Although a great improvement over the Metonic calendar, the Julian calendar was still not quite in synchronization with the seasons. The Venerable Bede, an English scholar who lived from 673-735, noted that the vernal equinox had slipped three days earlier than the traditional March 21. The Julian calendar remained in use, however, until replaced by the Gregorian calendar in the late sixteenth century. Although the Roman abbot Dionysius Exiguus proposed that the years be numbered from the birth of Christ in about 524 (Boyer 1968, p. 272), Bede was the first to actually date events from the birth of Christ. This system gives rise to the familiar classification of dates as BC or AD (also sometimes denoted BCE and CE). Interestingly enough, probably because the concept for zero was not widely used in Europe at the time, this method of dating omits the year zero, so that the year 1 BC is followed immediately by the year 1 AD. In any case, whoever zeroed the calendar made an error, since the Bible says Jesus was alive in Herod's time, but Roman records showed that Herod died in what turns out to be 4 BC.

The German astronomer Christoph Clavius (1537-1612) was the motivating force behind the needed revision of the Julian calendar. The reform brought the calendar back in synchronization with the seasons (which now occurred 11 days earlier that their traditional dates), and altered the rules under which leap years occurred. By the new rules, the years that were divisible by 400 were leap years, while other century years were not. These modifications were sufficient to match almost precisely the length of the tropical year.

The reform was first adopted by Pope Gregory XIII, who decreed that the day after October 4, 1582 would be October 15, 1582. This decree was followed by the Catholic countries of France, Spain, Portugal, and Italy. Various Catholic German countries (Germany was not yet unified), Belgium, the Netherlands, and Switzerland followed suit within a year or two, and Hungary followed in 1587. Because of the Pope's decree, the reform of the Julian calendar came to be known as the Gregorian calendar. The rest of Europe did not follow suit for more than a century. The Protestant German countries adopted the Gregorian reform in 1700. By this time, the calendar trailed the seasons by twelve days. England finally followed suit in 1752, declaring that Wednesday, September 2, 1752 was immediately followed by Thursday, September 14, 1752 as shown in the below calendar. The English calendar was also used in America.

English Calendar:

Sweden followed England's lead in 1753. Russia, however, did not follow suit until 1918, when January 31, 1918 was immediately followed by February 14th. In fact, Russia is not on the Gregorian calendar, but on a more accurate one of their own devising. The Russian calendar is designed to more closely approximate the true length of the tropical year, thus has one additional rule for when a year is a leap year. It will remain in synchronization with the Gregorian calendar for thousands more years, by which time one or both will have probably fallen into disuse. Similarly, Iranian calendar is also a more accurate version of the Gregorian calendar (Ross).

The names of the days of the week were derived from gods, "planets," and--in some languages--metals. These name were later carried over to almost all modern European languages, though the names may sound different. In English, Wednesday is derived from a form of the Norse god Odin and Thursday from the Norse God Thor.

During the French Revolution, the French invented and put into use a new French revolutionary calendar. The Revolutionary calendar was established in October 1793, but Year I was made effective on September 22, 1792 (the autumnal equinox). The Revolutionary calendar had 12 months of 30 days, plus 5 or 6 leap days (with a rule for leap years). The French Revolutionary calendar was abolished when Napoleon re-instituted the Gregorian calendar on December 31, 1805.

The Julian calendar still remains in some use, since it is the basis of the system of the Julian date, devised by Clavius' contemporary Julian Scaliger (1540-1609). (In addition, some religious sects still calculate holidays based on the Julian calendar.) The name for this system, incidentally, was from Julius Scaliger, not Julius Caesar. In it, Scaliger defined Day One was as a day when three cycles converged on it. The first cycle was the 28 year period over which the Julian calendar repeats. (After 28 years, all the dates fall on the same days of the week, so you need only buy 28 calendars. Note that since the Gregorian calendar was adopted the calendar now takes 400 years to repeat.) The second was the 19 year Metonic cycle, over which phases of the moon almost land on the same dates of the year. The third cycle was the 15 year ancient Roman tax cycle. Scaliger picked January 1, 4713 BC on the Julian calendar as Day One (Seidelmann 1992, p. 55). I don't know the significance for picking this date as opposed to any other "triple convergence" date.

After Julian date One, subsequent Julian dates are sequential. Therefore, midnight before January 1, 1982 is Julian Date 2,444,970.5. The modified Julian date system, defined as the Julian date minus 2,400,000.5, is also occasionally used by astronomers, but not so frequently in recent years. The Julian and Gregorian calendars differ by 13 days in the 20th and 21st Centuries. They would have been in synchronization during the 3rd Century.

The following table gives the dates corresponding to January 1, 1989 in the Gregorian calendar for various other calendar systems (Astronomical Almanac).

*begins at sunset

AD, Aztec Calendar, Babylonian Calendar, BC, BCE, Besselian Epoch, Chinese Calendar, CE, Egyptian Calendar, French Revolutionary Calendar, Gregorian Calendar, Hebrew Calendar, Hindu Calendar, Iranian Calendar, Islamic Calendar, Julian Calendar, Julian Date, Julian Epoch, Mayan Calendar, Roman Calendar, Time

References
Berlekamp, E. R.; Conway, J. H.; and Guy, R. K. Winning Ways for Your Mathematical Plays, Vol. 2: Games in Particular. New York: Academic Press, pp. 795-800, 1982.
Bickerman, E. J. Chronology of the Ancient World, rev. ed. London: Thames and Hudson, 1980.
Boyer, C. B. A History of Mathematics. New York: Wiley, 1968.
Dershowitz, N. and Reingold, E. M. Calendrical Calculations: The Millennium Edition. Cambridge, England: Cambridge University Press, 1999.
Doggett, L. E. "Calendars." Ch. 12 in Explanatory Supplement to the Astronomical Almanac (Ed. P. K. Seidelmann). Mill Valley, CA: University Science Books, pp. 575-608, 1992. http://astro.nmsu.edu/~lhuber/leaphist.html.
Duffett-Smith, P. "Calendars." §1 in Practical Astronomy with Your Calculator, 3rd ed. Cambridge, England: Cambridge University Press, pp. 1-2, 1992.
Gill, H. S. "About.com Ancient/Classical History." http://ancienthistory.about.com/education/ancienthistory/msub_calendar.htm.
Kraitchik, M. "The Calendar." Ch. 5 in Mathematical Recreations. New York: W. W. Norton, pp. 109-116, 1942.
Lee, S. E. "Calendar Conversions." http://genealogy.org/~scottlee/calconvert.cgi.
Linden, W. "Today's Calendar and Clock Page." http://www.panix.com/~wlinden/calendar.shtml
McLean, J. "CalendarLand." http://www.juneau.com/home/janice/calendarland/.
Neugebauer, O. The Exact Sciences in Antiquity, 2nd ed. New York: Dover, pp. 80-91, 1969.
Parise, F. (Ed.). The Book of Calendars. New York: Facts on File.
Ross, K. L. "Iranian Calendars." http://www.friesian.com/calendar.htm#iran.
Schocken, W. A. The Calculated Confusion of the Calendar. New York: Vantage Press, 1976.
Seidelmann, P. K. (Ed.). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books, 1992.
Stockton, J. "John Stockton's Date & Time Miscellany." http://www.merlyn.demon.co.uk/misctime.htm.
United States Government Printing Office. Astronomical Almanac for the Year 2000. Washington, DC: U. S. Government Printing Office, p. B2, 2000.
Vardi, I. "The Calendar." Ch. 3 in Computational Recreations in Mathematica. Reading, MA: Addison-Wesley, pp. 35-55, 1991.
Vardi, I. "Calendar.m." http://www.mathsource.com/cgi-bin/msitem?Enhancements.Other0200-776

Japanese Era

A New Calendrical System
Over the years, many different groups and individuals have attempted to redefine the way in which we humans keep track of passing years. Each group or person chooses a year in which to start numbering, and then the years follow consecutively, which is all well and good as far as that goes. There is, for example, the One True Discordian Calendar, which is naturally superior to all others. Also, Robert Anton Wilson describes his multi-cultural dating method in his essay, ``How to Live Eleven Days in 24 Hours''. His ideas are also naturally superior to all others, but Robert Anton Wilson is a nincompoop, and a bald-headed liar besides. Therefore, we reject his system, even if it isn't really a system at all, which it is. In any case, his essay describes many and varied year-numbering systems, so we won't repeat them here. Further, there are many dating systems which Wilson does not mention -- such as the Celtic Church reckoning, the Roman system, and the Julian calendar. There are certainly many more, too.
      But the important aspect of all of these dating systems is that they all involve numbers, which leads us to one seemingly inescapable problem, not with the calendars themselves, but with the human beings who use them, namely this:
      Humans have an obsession with round numbers.
      This leads people to attach importance to otherwise unimportant units of time larger than the year itself, which is nicely based on the Newtonian mechanics of the solar system and therefore has some basis in observable phenomena. Most notably, this penchant for round numbers causes people to regard human history in terms of decades, starting with years that end in one zero, and centuries, starting with years that end in double zeroes. (It should be noted though that in, for example, the Gregorian calendar with which most people are familiar, centuries and decades do not really start in the year with the zero on the end, but in the year with the one on the end, since the calendar was worked out by non-intelligent types who left out the year 0.) (The previous parenthetical statement may strike some as excessively picayune coming from an author with a known chaotic temperment. Consider that sometimes, chaos can appear very orderly, and that, even in a chaotic system, certain disorderly elements may get their ass kicked regardless. Metaphysically speaking, of course.)
      It occured to this author, thinking about human life spans, that the division of human time into centuries is an unfair and arbitrary split. It's not as if everyone who was alive in 1899 suddenly died in 1900 and was replaced by ``twentieth century'' people. But, when one looks at dates in terms of decades and centuries, that's the idea one gets. (At least, the idea this one gets.)
      ``Einstein, the greatest scientist of the twentieth century,'' a history book might say. Why not the greatest scientist of the years between 1853 and 1951? Why not the greatest scientist of not only his lifetime, but also of 1066?
      It annoys this author that, should he be discussing the 1977 release of Star Wars with twelve-year-old children in the Gregorian year 2015, they'll think it was a hundred years ago when it was really less than forty. There is an unnatural break that will most likely occur during this author's lifetime, after which he will be a relic of the ``twentieth century.''
      Fuck all that.
      This author has a better way. This author has the One True Dating System which will cure all these ills. It occurred to him that any counting system involving years requires some sort of ordering, and ordering implies these round numbers -- unless one can find an ordered system which does not immediately bring to mind those devils of digitization, those protagonists of parcelization, those ninny zeroes of base 10.
      And this author found one: The Alphabet.
      Certainly, the alphabet does have its own unnatural division; that is, when Z is followed by A, we've come full circle, and there is something of a break there. But this author finds this break much less of an obvious stake in the ground, much less of a chainlink fence across historical consciousness. (Can one speak of such a fence? Or should it evolve from excrement on a tree through piles of stones and hedgerows and split-rails up to electrified chainlink? It doesn't matter, the metaphor is being mangled anyway.)
      So, we can order the years A, B, C, and so on, eventually getting to Z, then following that with AA, AB, AC, and so on and so forth.
      This is all good, again as far as it goes. But now we need to find a starting date, the First Year from which we will start lettering. Different cultures have answered this question many ways. Discordians, for example, start their ordering from the Original Snub. Of course, this is the best spot, but nonetheless, we shall look for another. The Islamic calendar begins its numbering from the hegira, the date when Mohammed fled from Mecca. Clearly, this is silly, since hegira isn't even an English word and no one's really sure how to spell it. The Gregorian and Julian calendars place Year One at the birth of a probably fictional character named Jesus; and, to add even less palatable ingredients to an already gamey stew, people who actually believe in this person have placed his date of birth anywhere from four years to seventeen years previous to this Year One. Since not everyone can get together on this matter -- even those who claim to be on the same side -- this is obviously a bad choice for reckoning (even if your bank won't accept checks based on any other system).
      The ancient Roman system used the date of the founding of Rome. Unfortunately, all the anicent Romans are dead and have been replaced by people who willingly elect pornographic models to public office, and while they are to be commended for such activities, they certainly can't be trusted with important things like dates. The ancient Hebrew calendar began its counting with the victory of one Seleucus at Gaza, but he's not even Jewish. The Jews later calculated the creation of the world as having occured 3007 years prior to the founding of Rome and used that as their First Year. Any group which is going to change its mind on matters of such import shouldn't be allowed near a calendar, at least in this author's opinion.
      Others also have adopted their own numbering systems, based on egomania, James Joyce, the Bible, Hung Mung, and other fictions. All of these are as meaningless as anything else. (Except, of course, for this new system, which is deeply and importantly meaningful.)
      Fuck all that.
      According to Isaac Asimov in Of Time and Space and Other Things, the earliest event we can date exactly is a battle which was scheduled to occur between the Lydians and the Medians in Asia Minor. This battle was thoughtfully called off on account of total eclipse of the Sun, and, thanks to the modern invention of Newtonian mechanics, we can calculate the exact date and time on which this eclipse occurred. In the Gregorian calendar, this date is May 28, 585 B.C.
      This strikes the author as being a perfect date for Year A, since it's the earliest date we can be sure of. In fact, this author thinks it's a perfect date for Chaos 1, A -- the first day of the first year. (We might as well keep the Discordian months -- why not?)
      The author has chosen to call this wonderful system the Jusanotoron Calendar, for JUst SAy NO TO ROund Numbers. You can call it whatever you like. For example, you can decide that Jusanotoron is close enough to Joshua Norton to call this the Joshua Norton Memorial Reckoning. Or, you may just decide to call this system flapdoodle.
      Anyway.
      So, in the Jusanotoron Calendar, the fictional year of the fictional birth of the fictional character known as Jesus is VN. This year -- 1995 Gregorian -- is CUF. Today, in fact, is Confusion 47, CUF. Pearl Harbor was supposedly bombed on Confusion 48, CSD. And the fictional date of the fictional birth of the fictional character known as Pope Icky Fundament is Confusion 60, CTG.
      Keep an eye out for this dating system on official letterheads everywhere.
      So much for all that.

Kalendere i forskjellige kulturer

Angivelse av datoen 1. januar 2000 i ulike kalendere:

The Burroughsian Calendar: This is the dream calendar of William S. Burroughs. It began on 23 Terre Haute, 1 which corresponds to Julian Day Number - 2,440,579

The Poundian Calendar: This is based directly on the Gregorian calendar, merely changing the starting day and year. Designed by Ezra Pound, 1 Hephaistos, 1 psU (post scriptum Ulysses) was the day after Joyce finished writing Ulysses, thus marking the end of the Christian Era. This was also the day after Pound's 36^th birthday, and corresponds to Julian Day Number - 2,422,995

The Thelemic Calendar: Designed by Aleister Crowley when he received or conceived The Book of the Law and inaugurated the New Aeon of Horus, Anno 0 (Fool) : 0 (Fool) began on Julian Day Number - 2,416,560

The 'Pataphysical Calendar: Designed by Alfred Jarry, Sunday, 1 Absolu, 1 EP (Ere Pataphysique or ‘Pataphysical Era) marks his birth, corresponding to Julian Day Number - 2,405,410

The Bahá'í Calendar: Bahá Day of Bahá Month of Alif Year, 1 BE (Bahá'í Era) marks the first day of the year that the Bab started his ministry, corresponding to Julian Day Number - 2,394,647

The French Revolutionary Calendar: Introduced in France on Tridi, 3 Brumaire, An 2, marking the beginning of the first French Republic on the Autumnal Equinox as Primidi, 1 Vendemiaire, An 1 corresponding to Julian Day Number - 2,375,840

The Zodiac Calendar: I as yet have found no starting year to the cycle, if it were numbered at all.

The Wiccan Calendar: I as yet have found no starting year to the cycle, if it were numbered at all.

The Druidic Calendar: I as yet have found no starting year to the cycle, if it were numbered at all.

The Hopi Calendar: I as yet have found no starting year to the cycle, if it were numbered at all.

The Zoroastrian Calendar: Begun with the coronation of the last Zoroastrian Sasanian King, Yazdegird II, 1 Y (Yazdegird) corresponds to Julian Day Number - 1,951722

The Islamic Calendar: Begun with the flight of the prophet (Hijri), 1 Muharram, 1 AH (Anno Hegirae) corresponds to Julian Day Number - 1,948,437

The Jalaali Calendar: 1 farvardin, 1 corresponds to Julian Day Number - 1,948,323

The Indonesian Calendar: Decreed by Sultan Agung Hanyokrokosumo in 1585, year 1 corresonds to Julian Day Number - 1,750,000

This is the same year that the Hindu (civil) calendar begins on, but I don't know if it's a coincidence, because I don't know as yet what event(s) this correlates to, historical or astronomical.

The Gregorian Calendar: Proposed by Aloysius Lilius, a Naples physician, and decreed by Pope Gregory XIII in February, 1582 AD in a papal bull, this merely modified the use of leap-years in the Julian calendar, with a different starting year that had been established in 523 AD (see below). To realign the Vernal Equinox with 21 March, as it had been in 325 AD the year of the First Council of Nicaea, 10 days were dropped from October, 1582 AD. 1 January, 1 AD (Anno Domino or Year of the Lord) follows shortly after the supposed date of the birth of Jesus Christ, corresponding to Julian Day Number - 1,721,424

The date of Christ's birth was assumed to be 25 December, 1 BC (Before Christ) as established by Dionysius Exigus, a monk from Scythia, around 523 AD. How he calculated this is unknown, and it was disputed early on. Christ was born during the reign of King Herod, who died in 4 BC, and it is suggested the birth is actually around 7 BC.

The Babylonian Calendar: Year 1 corresponds to Julian Day Number - 1,448,007

The Julian Calendar: Designed by mathematician and astronomer Sosigenes, and enacted by Julius Caesar in 709 AUC to reform the old and confusing Roman calendar. 1 Januarius, 1 AUC (Ab Urbe Condita or Since the founding of Rome) corresponds to Julian Day Number - 1,446,427

The Erisian Calendar: This is based directly on the Gregorian calendar, merely changing the starting year. Designed by Malclypse the Younger, Sweetmorn, 1 Chaos, 1 YOLD (Year Of Our Lady Of Discord) marks the year of the Original Snub, as explained in The Principia Discordia, corresponding to Julian Day Number - 1,289,004 

The Chinese Calendar: The start of this calendar was a very rare day indeed! Shortly after dawn, the sun and new moon both aligned with the conjunction (within a few degrees of each other) of Mercury, Venus, Mars, Jupiter and Saturn in the constellation of Pegasus. This date began jia-zi 1 (Year of the Mouse) and corresponds to Julian Day Number - 1,007,826

The Tibetan Calendar: The calendar begins with the enlightening day of Buddha, which had a complete moon eclipse. Different Tibetan schools vary on this date, so I will use the oldest of the four I have and refer to it as Fire Rabbit 1. The other start dates appear in this system as Wood Horse 1108, Fire Bird 1591 and Water Dragon 2006. Fire Rabbit 1 corresponds to Julian Day Number - 942,097

The Balinese Calendar: I as yet have found no starting year to the cycles.

The Hindu Calendar: Established by The Calendar Reform Committee in 1879 SE, 1 Chaitra, 1 SE (Sata Era) in the civil calendar, corresponds to Julian Day Number - 1,750,037

This is the same year that the Indonesian calendar begins on, but I don't know if it's a coincidence, because I don't know as yet what event(s) this correlates to, historical or astronomical. Of the other Eras, the oldest of the starting dates is 1 KY (Kali Yuga or Iron Age), corresponding to Julian Day Number - 588,813

The Aztec Calendar: I haven't done much with this one yet, since it's so much like the Mayan, and that one has so much. Once I have the Mayan organized, then it will probably be easier to sort this one out as well.

The Mayan Calendar: The beginning of the last Great Cycle corresponds to Julian Day Number - 584,298

The Hebrew Calendar: According to Jewish teachings, 1 Tishrei, 1 AM (Anno Mundane, or Year Of The World) follows the creation of the universe by God, corresponding to Julian Day Number - 347,998

The Illuminati Calendar: This is based directly on the Gregorian calendar, merely changing the starting year. Designed by Robert Anton Wilson in 5969-5971 AL for the Illuminatus trilogy (co-written with Bob Shea), 1 Verwirrung, 1 AL (Anno Lumina) marks the birth of ancient Chinese Chaoist (pre-Taoist) Hung Mung, corresponding to Julian Day Number - 260,460

The Egyptian Calendar: The cycle began when the sun and Sirius rose in the same place, which may have been one of three possibilities. Referring to the older as year 1, the second would appear in this system as year 1141 and the third as 1468. Year 1 corresponds to Julian Day Number - 172,609 I've been informed that this year 1 was debunked by Richard Parker in the 6190's, and that the currently accepted starting date is the 1141, but whenever I have multiple sources I refer to the oldest in my calculations, and reference others relative to it. 

The Julian Day Number: Developed by the French scholar Joseph Scaliger, this is simply the number of solar days elapsed since noon GMT on the first day of the cycle. The cycle began on a day in the Julian calendar when the Indiction, Golden Number, and Solar Number were all 1, corresponding to Julian Day Number - 0

1999 GPS Calendar

This calendar will help you convert a typical calendar day to either the Day of Year or GPS Week #. For example, July 6, 1999 is day of year 187 in GPS Week 1017.
The GPS Week # would be 10172 (the # 2 represents Tuesday.)
Sunday=0, Monday=1, Tuesday=2, Wednesday=3, Thursday=4, Friday=5, Saturday=6

A Summary of the International Standard Date and Time Notation

by Markus Kuhn

International Standard ISO 8601 specifies numeric representations of date and time. This standard notation helps to avoid confusion in international communication caused by the many different national notations and increases the portability of computer user interfaces. In addition, these formats have several important advantages for computer usage compared to other traditional date and time notations. The time notation described here is already the de-facto standard in almost all countries and the date notation is becoming increasingly popular.

Especially authors of Web pages and software engineers who design user interfaces, file formats, and communication protocols should be familiar with ISO 8601.

Contents: Date, Time of Day, Time Zone.

NEWS: The second edition ISO 8601:2000 has been published!

Date

The international standard date notation is

YYYY-MM-DD

where YYYY is the year in the usual Gregorian calendar, MM is the month of the year between 01 (January) and 12 (December), and DD is the day of the month between 01 and 31.

For example, the fourth day of February in the year 1995 is written in the standard notation as

1995-02-04

Other commonly used notations are e.g. 2/4/95, 4/2/95, 95/2/4, 4.2.1995, 04-FEB-1995, 4-February-1995, and many more. Especially the first two examples are dangerous, because as both are used quite often in the U.S. and in Great Britain and both can not be distinguished, it is unclear whether 2/4/95 means 1995-04-02 or 1995-02-04. The date notation 2/4/5 has at least six reasonable interpretations (assuming that only the twentieth and twenty-first century are reasonable candidates in our life time).

Advantages of the ISO 8601 standard date notation compared to other commonly used variants:

As dates will look a little bit strange anyway starting with 2000-01-01 (e.g. like 1/1/0), it has been suggested that the year 2000 is an excellent opportunity to change to the standard date notation.

ISO 8601 is only specifying numeric notations and does not cover dates and times where words are used in the representation. It is not intended as a replacement for language-dependent worded date notations such as "24. Dezember 2001" (German) or "February 4, 1995" (US English). ISO 8601 should however be used to replace notations such as "2/4/95" and "9.30 p.m.".

Apart from the recommended primary standard notation YYYY-MM-DD, ISO 8601 also specifies a number of alternative formats for use in applications with special requirements. All of these alternatives can easily and automatically be distinguished from each other:

The hyphens can be omitted if compactness of the representation is more important than human readability, for example as in

19950204

For situations where information about the century is really not required, a 2-digit year representation is available:

95-02-04 or 950204

If only the month or even only the year is of interest:

1995-02 or 1995

In commercial and industrial applications (delivery times, production plans, etc.), especially in Europe, it is often required to refer to a week of a year. Week 01 of a year is per definition the first week that has the Thursday in this year, which is equivalent to the week that contains the fourth day of January. In other words, the first week of a new year is the week that has the majority of its days in the new year. Week 01 might also contain days from the previous year and the week before week 01 of a year is the last week (52 or 53) of the previous year even if it contains days from the new year. A week starts with Monday (day 1) and ends with Sunday (day 7). For example, the first week of the year 1997 lasts from 1996-12-30 to 1997-01-05 and can be written in standard notation as

1997-W01 or 1997W01

The week notation can also be extended by a number indicating the day of the week. For example, the day 1996-12-31, which is the Tuesday (day 2) of the first week of 1997, can also be written as

1997-W01-2 or 1997W012

for applications like industrial planning where many things like shift rotations are organized per week and knowing the week number and the day of the week is more handy than knowing the day of the month.

An abbreviated version of the year and week number like

95W05

is sometimes useful as a compact code printed on a product that indicates when it has been manufactured.

The ISO standard avoids explicitly stating the possible range of week numbers, but this can easily be deduced from the definition:

Theorem: Possible ISO week numbers are in the range 01 to 53. A year always has a week 52. (There is one historic exception: the year in which the Gregorian calendar was introduced had less than 365 days and less than 52 weeks.)

Proof: Per definition, the first week of a year is W01 and consequently days before week W01 belong to the previous year and so there is no week with lower numbers. Considering the highest possible week number, the worst case is a leap year like 1976 that starts with a Thursday, because this keeps the highest possible number of days of W01 in the previous year, i.e. 3 days. In this case, the Sunday of W52 of the worst case year is day number 4+51*7=361 and 361-366=5 days of W53 belong still to this year, which guarantees that in the worst case year day 4 (Thursday) of W53 is not yet in the next year, so a week number 53 is possible. For example, the 53 weeks of the worst case year 1976 started with 1975-12-29 = 1976-W01-1 and ended with 1977-01-02 = 1976-W53-7. On the other hand, considering the lowest number of the last week of a year, the worst case is a non-leap year like 1999 that starts with a Friday, which ensures that the first three days of the year belong to the last week of the previous year. In this case, the Sunday of week 52 would be day number 3+52*7=367, i.e. only the last 367-365=2 days of the W52 reach into the next year and consequently, even a worst case year like 1999 has a week W52 including the days 1999-12-27 to 2000-01-02. q.e.d.

[The new 1999 version of the C programming language standard (ISO 9899) added in the strftime() function means to generate the ISO 8601 week notation. The author of this text developed a further proposal for a modernised clock and calendar API for C, which provides full proper treatment of leap seconds and timezones and fixes numerous other problems in the current C timing library functions. It also serves as a model for those who want to design clock library functions for other programming languages.]

Both day and year are useful units of structuring time, because the position of the sun on the sky, which influences our lives, is described by them. However the 12 months of a year are of some obscure mystic origin and have no real purpose today except that people are used to having them (they do not even describe the current position of the moon). In some applications, a date notation is preferred that uses only the year and the day of the year between 001 and 365 (366 in leap years). The standard notation for this variant representing the day 1995-02-04 (that is day 035 of the year 1995) is

1995-035 or 1995035

Leap years are years with an additional day YYYY-02-29, where the year number is a multiple of four with the following exception: If a year is a multiple of 100, then it is only a leap year if it is also a multiple of 400. For example, 1900 was not a leap year, but 2000 is one.

Time of Day

The international standard notation for the time of day is

hh:mm:ss

where hh is the number of complete hours that have passed since midnight (00-24), mm is the number of complete minutes that have passed since the start of the hour (00-59), and ss is the number of complete seconds since the start of the minute (00-60). If the hour value is 24, then the minute and second values must be zero. [The value 60 for ss might sometimes be needed during an inserted leap second in an atomic time scale like Coordinated Universal Time (UTC). A single leap second 23:59:60 is inserted into the UTC time scale every few years as announced by the International Earth Rotation Service in Paris to keep UTC from wandering away more than 0.9 s from the less constant astronomical time scale UT1 that is defined by the actual rotation of the earth.]

An example time is

23:59:59

which represents the time one second before midnight.

As with the date notation, the separating colons can also be omitted as in

235959

and the precision can be reduced by omitting the seconds or both the seconds and minutes as in

23:59, 2359, or 23

It is also possible to add fractions of a second after a decimal dot or comma, for instance the time 5.8 ms before midnight can be written as

23:59:59.9942 or 235959.9942

As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time:

1995-02-04 24:00 = 1995-02-05 00:00

In case an unambiguous representation of time is required, 00:00 is usually the preferred notation for midnight and not 24:00. Digital clocks display 00:00 and not 24:00.

ISO 8601 does not specify, whether its notations specify a point in time or a time period. This means for example that ISO 8601 does not define whether 09:00 refers to the exact end of the ninth hour of the day or the period from 09:00 to 09:01 or anything else. The users of the standard must somehow agree on the exact interpretation of the time notation if this should be of any concern.

If a date and a time are displayed on the same line, then always write the date in front of the time. If a date and a time value are stored together in a single data field, then ISO 8601 suggests that they should be separated by a latin capital letter T, as in 19951231T235959.

A remark for readers from the U.S.:

A remark for readers from German speaking countries:

In May 1996, the German standard DIN 5008, which specifies typographical rules for German texts written on typewriters, has been updated. The old German numeric date notations DD.MM.YYYY and DD.MM.YY have been replaced by the ISO date notations YYYY-MM-DD and YY-MM-DD. Similarly, the old German time notations hh.mm and hh.mm.ss have been replaced by the ISO notations hh:mm and hh:mm:ss. Those new notations are now also mentioned in the latest edition of the Duden. The German alphanumeric date notation continues to be for example "3. August 1994" or "3. Aug. 1994". The corresponding Austrian standard has already used the ISO 8601 date and time notations before.

ISO 8601 has been adopted as European Standard EN 28601 and is therefore now a valid standard in all EU countries and all conflicting national standards have been changed accordingly.

Time Zone

Without any further additions, a date and time as written above is assumed to be in some local time zone. In order to indicate that a time is measured in Universal Time (UTC), you can append a capital letter Z to a time as in

23:59:59Z or 2359Z

[The Z stands for the "zero meridian", which goes through Greenwich in London, and it is also commonly used in radio communication where it is pronounced "Zulu" (the word for Z in the international radio alphabet). Universal Time (sometimes also called "Zulu Time") was called Greenwich Mean Time (GMT) before 1972, however this term should no longer be used. Since the introduction of an international atomic time scale, almost all existing civil time zones are now related to UTC, which is slightly different from the old and now unused GMT.]

The strings

+hh:mm, +hhmm, or +hh

can be added to the time to indicate that the used local time zone is hh hours and mm minutes ahead of UTC. For time zones west of the zero meridian, which are behind UTC, the notation

-hh:mm, -hhmm, or -hh

is used instead. For example, Central European Time (CET) is +0100 and U.S./Canadian Eastern Standard Time (EST) is -0500. The following strings all indicate the same point of time:

12:00Z = 13:00+01:00 = 0700-0500

There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. In addition, politicians enjoy modifying the rules for civil time zones, especially for daylight saving times, every few years, so the only really reliable way of describing a local time zone is to specify numerically the difference of local time to UTC. Better use directly UTC as your only time zone where this is possible and then you do not have to worry about time zones and daylight saving time changes at all.