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Thursday, May 14, 2009

History Of Calenders Part - 4

When did the 3rd millennium start?

The first millennium started in AD 1, so the millennia are counted in this manner:
1st millennium: 1-1000
2nd millennium: 1001-2000
3rd millennium: 2001-3000
Thus, the 3rd millennium and, similarly, the 21st century started on 1 Jan 2001.
This is the cause of some heated debate, especially since some dictionaries and encyclopedias say that a century starts in years that end in 00. Furthermore, the change 1999/2000 is obviously much more spectacular than the change 2000/2001.
Let us propose a few compromises:
Any 100-year period is a century. Therefore the period from 23 June 2004 to 22 June 2104 is a century. So please feel free to celebrate the start of a century any day you like!
Although the 20th century started in 1901, the 1900s started in 1900. Similarly, the 21st century started in 2001, but the 2000s started in 2000.

What do A.D., B.C., C.E., and B.C.E. stand for?

Years before the birth of Christ are in English traditionally identified using the abbreviation B.C. ("Before Christ").
Years after the birth of Christ are traditionally identified using the Latin abbreviation AD ("Anno Domini", that is, "In the Year of the Lord").
Some people, who want to avoid the reference to Christ that is implied in these terms, prefer the abbreviations BCE ("Before the Common Era" or "Before the Christian Era") and CE ("Common Era" or "Christian Era").

Historical eras & chronology

The calendars described in this exhibit, except for the Chinese calendar, have counts of years from initial epochs. In the case of the Chinese calendar and some calendars not included here, years are counted in cycles, with no particular cycle specified as the first cycle. Some cultures eschew year counts altogether but name each year after an event that characterized the year. However, a count of years from an initial epoch is the most successful way of maintaining a consistent chronology. Whether this epoch is associated with an historical or legendary event, it must be tied to a sequence of recorded historical events.
This is illustrated by the adoption of the birth of Christ as the initial epoch of the Christian calendar. This epoch was established by the sixth-century scholar Dionysius Exiguus, who was compiling a table of dates of Easter. An existing table covered the nineteen-year period denoted 228-247, where years were counted from the beginning of the reign of the Roman emperor Diocletian. Dionysius continued the table for a nineteen-year period, which he designated Anni Domini Nostri Jesu Christi 532-550. Thus, Dionysius’ Anno Domini 532 is equivalent to Anno Diocletian 248. In this way a correspondence was established between the new Christian Era and an existing system associated with historical records. What Dionysius did not do is establish an accurate date for the birth of Christ. Although scholars generally believe that Christ was born some years before A.D. 1, the historical evidence is too sketchy to allow a definitive dating.
Given an initial epoch, one must consider how to record preceding dates. Bede, the eighth-century English historian, began the practice of counting years backward from A.D. 1 (see Colgrave and Mynors, 1969). In this system, the year A.D. 1 is preceded by the year 1 B.C.E., without an intervening year 0. Because of the numerical discontinuity, this "historical" system is cumbersome for comparing ancient and modern dates. Today, astronomers use +1 to designate A.D. 1. Then +1 is naturally preceded by year 0, which is preceded by year -1. Since the use of negative numbers developed slowly in Europe, this "astronomical" system of dating was delayed until the eighteenth century, when it was introduced by the astronomer Jacques Cassini (Cassini, 1740).
Even as use of Dionysius’ Christian Era became common in ecclesiastical writings of the Middle Ages, traditional dating from regnal years continued in civil use. In the sixteenth century, Joseph Justus Scaliger tried to resolve the patchwork of historical eras by placing everything on a single system (Scaliger, 1583). Instead of introducing negative year counts, he sought an initial epoch in advance of any historical record. His numerological approach utilized three calendrical cycles: the 28-year solar cycle, the nineteen-year cycle of Golden Numbers, and the fifteen-year indiction cycle. The solar cycle is the period after which weekdays and calendar dates repeat in the Julian calendar. The cycle of Golden Numbers is the period after which moon phases repeat (approximately) on the same calendar dates. The indiction cycle was a Roman tax cycle. Scaliger could therefore characterize a year by the combination of numbers (S,G,I), where S runs from 1 through 28, G from 1 through 19, and I from 1 through 15. Scaliger noted that a given combination would recur after 7980 (= 28*19*15) years. He called this a Julian Period, because it was based on the Julian calendar year. For his initial epoch Scaliger chose the year in which S, G, and I were all equal to 1. He knew that the year 1 B.C.E. was characterized by the number 9 of the colar cycle, by the Golden Number 1, and by the number 3 of the indiction cycle, i.e., (9,1,3). He found that the combination (1,1,1) occurred in 4713 B.C.E. or, as astronomers now say, -4712. This serves as year 1 of Scaliger’s Julian Period. It was later adopted as the initial epoch for the Julian day numbers.

ISO 8601

What date format does the Standard mandate?

There are three basic formats: Calendar date, ordinal date, and week date.
A calendar date should be written as a 4-digit year number, followed by a 2-digit month number, followed by a 2-digit day number. Thus, for example, 2 August 1953 may be written:
19530802 or 1953-08-02
An ordinal date should be written as a 4-digit year number, followed by a 3-digit number indicating the number of the day within the year. Thus, for example, 2 August 1953 may be written:
1953214 or 1953-214
2 August is the 214th day of a non-leap year.
A week date should be written as a 4-digit year number, followed by a W, followed by a 2-digit week number followed by a 1-digit week day number (1=Monday, 2=Tuesday, ..., 7=Sunday). The week number is defined in section 7.7. Thus, for example, 2 August 1953 may be written:
1953W317 or 1953-W31-7
2 August was the Sunday of week 31 of 1953.
In all the examples above, the hyphens are optional.
Note that you must always write all the digits. Thus the year 47 must be written as 0047.

What time format does the Standard mandate?

A 24-hour clock must be used. A time is written as a 2-digit hour, followed by a 2-digit minute, followed by a 2-digit second, followed by a comma, followed by a number of digits indicating a fraction of a second. For example, thus:
140812,35 or 14:08:12,35
The fraction, the seconds, and the minutes may be omitted if less accuracy is required:
140812 or 14:08:12
1408 or 14:08
In all the examples above, the colons are optional. The comma may be replaced by a period (.), but this is not recommended.
The time may optionally be followed by a time zone indication. For UTC, the time zone indication is the letter Z. For other time zones, the indication is a plus or minus followed by the time difference to UTC (plus for times east of Greenwich, minus for times west of Greenwich). For example:
1130Z (11:30 UTC)
1130+0430 (11:30, at a location 4 and a half hours ahead of UTC)
1130-05 (11:30, at a location 5 hours behind of UTC)

What if I want to specify both a date and a time?

Date and time indications can be strung together by putting the letter T between them. For example, ten minutes to 7 p.m. on 2 August 1953 may be written as:
19530802T185000 or 1953-08-02T18:50:00

What format does the Standard mandate for a time interval?

There are several to choose from. A time interval can be specified as a starting time and an ending time or as a duration together with either a starting time and an ending time.
There are too many details to cover, so here are a few examples:
Using starting time and ending time:
Using starting time and duration:
This last example should be read as the time interval starting on 12 March 1927 at 08:04 and lasting for 1 year, 4 months, 12 days, 6 hours, 30 minutes, and 9 seconds. The letter P following the slash indicates that a duration follows.

Can I write BC dates and dates after the year 9999 using ISO 8601?

Yes, you can.
The year 1 BC must be written as 0000. The year 2 BC must be written as -0001, the year 3 BC must be written as -0002 etc.
Years of more than 4 digits must be written with an initial plus sign. Thus the year AD 10000 must be written as +10000.

Can I write dates in the Julian calendar using ISO 8601?

No. The Standard requires that the Gregorian calendar be used for all dates. Dates before the introduction of the Gregorian calendar are written using the proleptic Gregorian calendar. This is one of the few places where the proleptic Gregorian calendar is used.
Thus the Julian date 12 March 826 must be written as 0826-03-16, because its equivalent date in the Gregorian calendar is 16 March.

Does the Standard define the Gregorian calendar?

Yes, ISO 8601 specifies how the Gregorian calendar works. The specification is completely compatible with the calendar specified by Pope Gregory XIII in 1582, except that ISO 8601 does not concern itself with the calculation of Easter.
However, the calendar reference point used by the Standard is not Christ’s birth but the date on which the metric convention ("Convention du Metre") was signed in Paris. The Standard defines that date to be 20 May 1875.
Similarly, the reference point of the week cycles is 1 January 2000, which is defined to be a Saturday.
Of course, these reference points are also completely compatible with common usage.

What does the Standard say about the week?

According to ISO 8601, Monday is the first day of the week.
Each week has a number. A week that lies partly in one year and partly in another is assigned a number in the year in which most of its days lie. The Standard specifies this by saying that week 1 of any year is the week that includes the first Thursday of that year.

Why are ISO 8601 dates not used in this Calendar FAQ?

The Standard specifies how to write dates using only numbers. The Standard explicitly does not cover the cases where dates are written using words (such as January, February, etc.). In fact, the Standard itself makes frequent use of dates such as "20 May 1875" and "15 October 1582".
In other words, ISO 8601 helps people with data communication where it is natural to use all-number dates. In everyday language (spoken and written) we are free to use the terms we like best.

Where can I get the Standard?

If you are looking for a free copy somewhere on the internet, forget it! ISO makes money from selling copies of their standards.
ISO 8601:2004 can be bought from ISO at http://www.iso.ch. It is very expensive. The last time we checked, the price was 126 Swiss Francs (about U.S. $103) for a 33 page document.
Your local library may be able to find a copy for you.

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