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ISO8601@yahoogroups.com, NGUYEN Adam <adam917@s...> wrote:

>I heard that the Persian calendar is one of the most complex ever,

> going out-of-sync by one day every so many million years!

At one level, it is not very complex at all. It is a 365/366 day

calendar. The first six months have a 31 day length, the next six 30

in a leap year. The last month is 29 days in non-leap year. The first

day of the year is the day of the spring equinox IF it occurs before

noon, Tehran time, otherwise, the next day. This makes it self-

correcting until the average length of the year decreases to <365

days. The only cumbersome part is depending on calculating exactly

the time of the equinox. They use a "best science" approach and all

known perturbation terms plus leap seconds need to be considered if

the equinox is close to noon. If it further from noon, simpler

approximations work.

These can be calculated some distance ahead. As the calculation is

cumbersome, people try to fit it with numerical series (the most

common is a 33 year cycle) but these breakdown over time (somebody

else has a 2820 year cycle). Here is a link to one approach:

http://www.projectpluto.com/calendar.htm#jalali
The fact that it only varies plus or minus one from Gregorian

(because leap years are intercalated differently) for a LONG time

says rigid rules work too. We are approaching a point where the

Gregorian calendar should really switch from a 400 year long cycle to

a 500 year. If the leap day in 2400 were deferred to 2500, we could

use that cycle to around the year 8000.