# Calendar conversion theory

1. Jun 7, 2007

### lewis198

Conversion of calendars

Hey guys do you know where I could get info on calendar conversion theory? Including all the equations?

2. Jun 7, 2007

### Staff: Mentor

It isn't just a set of algorithms. What exactly are you interested in doing?
Do you understand that, for example, some calendars underwent reformation at different times in different countries - the Julian - Gregorian is one.

A book (minus some problems about year counting with regard to the Chinese calendar) has a discussion on 40 calendars and conversions between all of them with supporting C code:
Latham, Lance, 1995 'Standard C Date/Time Library: Programming the World's Calendars and Clocks '

Calendars are more about history, politics, religion and early astronomy
than anything else.

All calendric conversions work this basic way:
Convert the date in calendar A to JD (Julian days)
Convert JD to calendar B date.

Not all conversions are valid all the time. My experience is that there are good resources for some conversions, and conflicting resources for others.

3. Jun 7, 2007

### Chris Hillman

Can you be more specific? Julian calendar vs Gregorian calendar? Mayan calendar vs "modern" calendars? This could quickly get very tricky since conversion from ancient calendars typically involves archaeological dating controversies as well as astronometry. (E.g. controversies abound in Minoan and Egyptian chronologies.)

If you are primarily interested in Western calendars, some books on the theory of algorithms describe conversion algorithms.

For some interesting mathematics, you might look into Coll charts for truly relativistic satellite navigation systems (see the website in my sig and use the search tool). Current proposals for next generation satellite navigation systems really get at the heart of what we mean by timekeeping in a very interesting way, in IMO constitute a true revolution in intellectual history. A sweeping statement, I know, but check it out!

Sadly, I feel that I must add a caveat: calendar systems have always been one of those areas that attract an unusual number of cranks. Ditto archaeological controversies. So on the web you can find quite a few calendar cranks, so be careful that your sources are reliable.

Last edited: Jun 7, 2007
4. Jun 8, 2007

### lewis198

Thanks for your reply, I was interested in conversions between Babylonian, Julian and Egyptian calendars. Do you know if formulas exist that provides just that?

5. Jun 8, 2007

### lewis198

Thanks for your reply, I was interested in conversions between Babylonian, Julian and Egyptian calendars. Do you know if formulas exist that provides just that?

6. Jun 8, 2007

### Staff: Mentor

Yes, Latham's book explains those. And all his assumptions about how his versions work. But like I mentioned, it is not a one-to-one deal. There are different starting points for the Middle Egyptian calendar, for example. It supposedly works on a "heliacal cycle" of ~1460 years, and is also based in part on the flooding of the Nile. Therefore, a lot of dates you can come up with from other calendars may not make sense. You use different formulas depending on which starting point you choose, for example.

This may sound odd, but calendars are 'consumer items', perfect only as defined by the person who understands the one being used. If this is for even slightly serious work you should consider doing a lot of reading.

Just like in Math, as long as you define your initial assumptions and then say date y (in calendar system A)= date x (in calendar system B) you are okay.
Calendars can be an endless source of debate IMO.

http://en.wikipedia.org/wiki/Julian_calendar

http://en.wikipedia.org/wiki/Egyptian_calendar
Some of the ideas here seem to disagree with some the assumptions Latham uses (Sothis rising cyle thing). I am not saying who is right or wrong because I don't know.

Last edited: Jun 8, 2007
7. Jun 8, 2007

### Hurkyl

Staff Emeritus

8. Jun 8, 2007