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Documentation Frequently Asked Questions About Calendars



1. What Astronomical Events Form the Basis of Calendars?
1.1. What are equinoxes and solstices?
2. The Christian Calendar
2.1. What is the Julian calendar?
2.1.1. What years are leap years?
2.1.2. What consequences did the use of the Julian
calendar have?
2.2. What is the Gregorian calendar?
2.2.1. What years are leap years?
2.2.2. Isn't there a 4000-year rule?
2.2.3. Don't the Greeks do it differently?
2.2.4. When did country X change from the Julian to
the Gregorian calendar?
2.3. What day is the leap day?
2.4. What is the Solar Cycle?
2.5. What is the Dominical Letter?
2.6. What day of the week was 2 August 1953?
2.7. When can I reuse my 1992 calendar?
2.8. What is the Roman calendar?
2.7.1. How did the Romans number days?
2.9. What is the proleptic calendar?
2.10. Has the year always started on 1 January?
2.11. Then what about leap years?
2.12. What is the origin of the names of the months?
2.13. What is Easter?
2.13.1. When is Easter? (Short answer)
2.13.2. When is Easter? (Long answer)
2.13.3. What is the Golden Number?
2.13.4. How does one calculate Easter then?
2.13.5. What is the Epact?
2.13.6. How does one calculate Gregorian Easter then?
2.13.7. Isn't there a simpler way to calculate Easter?
2.13.8. Isn't there an even simpler way to calculate
2.13.9. Is there a simple relationship between two
consecutive Easters?
2.13.10. How frequently are the dates for Easter repeated?
2.13.11. What about Greek Orthodox Easter?
2.13.12. Did the Easter dates change in 2001?
2.14. How does one count years?
2.14.1. How did Dionysius date Christ's birth?
2.14.2. Was Jesus born in the year 0?
2.14.3. When does the 3rd millennium start?
2.14.4. What do AD, BC, CE, and BCE stand for?
2.15. What is the Indiction?
2.16. What is the Julian period?
2.16.1. Is there a formula for calculating the Julian
day number?
2.16.2. What is the modified Julian day number?
2.16.3. What is the Lilian day number?
2.17. What is the correct way to write dates?
3. ISO 8601
3.1. What date format does the Standard mandate?
3.2. What time format does the Standard mandate?
3.3. What if I want to specify both a date and a time?
3.4. What format does the Standard mandate for a time
3.5. Can I write BC dates and dates after the year 9999
using ISO 8601?
3.6. Can I write dates in the Julian calendar using ISO 8601?
3.7. Does the Standard define the Gregorian calendar?
3.8. What does the Standard say about the week?
3.9. Why are ISO 8601 dates not used in this Calendar FAQ?
3.10. Where can I get the Standard?
4. The Hebrew Calendar
4.1. What does a Hebrew year look like?
4.2. What years are leap years?
4.3. What years are deficient, regular, and complete?
4.4. When is New Year's day?
4.5. When does a Hebrew day begin?
4.6. When does a Hebrew year begin?
4.7. When is the new moon?
4.8. How does one count years?
5. The Islamic Calendar
5.1. What does an Islamic year look like?
5.2. So you can't print an Islamic calendar in advance?
5.3. How does one count years?
5.4. When will the Islamic calendar overtake the Gregorian
5.5. Doesn't Saudi Arabia have special rules?
6. The Persian Calendar
6.1. What does a Persian year look like?
6.2. When does the Persian year begin?
6.3. How does one count years?
6.4. What years are leap years?
7. The Week
7.1. What is the origin of the 7-day week?
7.2. What do the names of the days of the week mean?
7.3. What is the system behind the planetary day names?
7.4. Has the 7-day week cycle ever been interrupted?
7.5. Which day is the day of rest?
7.6. What is the first day of the week?
7.7. What is the week number?
7.8. How can I calculate the week number?
7.9. Do weeks of different lengths exist?
8. The French Revolutionary Calendar
8.1. What does a Republican year look like?
8.2. How does one count years?
8.3. What years are leap years?
8.4. How does one convert a Republican date to a Gregorian one?
9. The Maya Calendar
9.1. What is the Long Count?
9.1.1. When did the Long Count start?
9.2. What is the Tzolkin?
9.2.1. When did the Tzolkin start?
9.3. What is the Haab?
9.3.1. When did the Haab start?
9.4. Did the Mayas think a year was 365 days?
10. The Chinese Calendar
10.1. What does the Chinese year look like?
10.2. What years are leap years?
10.3. How does one count years?
10.4. What is the current year in the Chinese calendar?
11. Frequently Asked Questions about this FAQ
11.1. Why doesn't the FAQ describe calendar X?
11.2. Why doesn't the FAQ contain information X?
11.3. Why don't you reply to my e-mail?
11.4. How do I know that I can trust your information?
11.5. Can you recommend any good books about calendars?
11.6. Do you know a web site where I can find information
about X?
12. Date


This is code that correctly calculates easter from the given year. It's based on 'ten divisions that lead to easter' in 'puzzles and paradoxes' by tim obrein. It is completely correct. [i wrote it because i needed to find easter on any given year when calculating pernsonnel data. You type in 'easter 1944' and it runs it under QBASIC (which is in Windows NT4), and CMD.EXE. Because you can't pass params to QBASIC scripts, we fetch it from the environment.

-------EASTER.BAT ---------
@echo off
set qbascmd=%1
c:\save\dos\qbasic /run L:\save\cdata\batch\easter.bas
set qbascmd=
DEF fnddv (y, x) = INT(y / x)
yr = VAL(s$)
bb = fnddv(yr, 100): cc = yr MOD 100: aa = yr MOD 19
dd = fnddv(bb, 4): ee = bb MOD 4
gg = fnddv(8 * bb + 13, 25)
hh = (19 * aa + bb - dd - gg + 15) MOD 30
gm = fnddv(aa + 11 * hh, 319)
ii = fnddv(cc, 4): kk = cc MOD 4
gl = (2*ee+2*ii-kk-hh+gm+32) mod 7
t1 = hh - gm + gl + 19
nn = fnddv(t1+71, 25)
pp = (t1 + nn) mod 32
print using "Easter Sunday of #### is ##/##"; yr, pp, nn,

Of course, even though they went with epacts (age of the moon at 1 march), the other way of adjusting the calendar is the method of golden jumps (you add an extra 8 for every 360+ years that have elapsed. This means that the new moon is moved onto the next node that would occur, the node after 1 is 9, so if one supposes that one is going from 19 to 1, a golden jump would make it end at 9. The easter full moon then still falls on the 19 traditional days, but the golden number is not Y mod 19, but (Y div 360 * 8 + year) mod 19.