-=> Quoting Eef Hartman to Melanie Baillot <=-

 EH> Melanie Baillot (1:163/215) schreef aan Arun Jayaprakash op 01-Mar-99
 EH> om 19:27: 
 MB> Can someone explain why they just can't do it from 1000 to 9999 

 EH> You still have only about 100 years range in the year (it IS only 7
 EH> bits, so a 0-127 value). 
 MB> ?? There must be some mathematical thing to calculate at what day 
 MB> the begining of a year starts etc...

 EH> Not really, especially with the various (in different countries)
 EH> calendar reforms. Essentially you just calculate out the number of
 EH> days difference (modulus 7) between the date you want the day OF and a
 EH> reference date you KNOW what day it was. In this you have to account
 EH> for leap years etc. And, of course, the "being a leap year" of the
 EH> centuries depend on which country you are doing it for, the calendar
 EH> reform in which 1700/1800/1900 became NON-leap years was proclaimed
 EH> (by the pope) in the 16th century, but Protestant countries like the
 EH> UK and the USA didn't implement it until the 18th and for instance
 EH> Russia only implemented it this century. So in France and Italy 1700
 EH> was NOT a leap year but in the USA it was. 

Actually, this is only a problem in some pretty rare cases. Most of the
time, you just accepta the fact that your date is Gregorian, even
though the country may still have been on Julian at the time. The day
of the week *will* be the same. And if you need the Julian calanedar
date, that's *really* easy to calculate for any given date:


year  Julian
----  ------
   1  -3
 100  -2
 200  -1
 300  0
 400  0
 500  +1
 600  +2
 700  +3
 800  +3
 900  +4
1000  +5
1100  +6
1200  +6
1300  +7
1400  +8
1500  +9
1600  +10
1700  +11
1800  +12
1900  +13
2000  +13
2100  +14

The only time/place I know of where neither the Julian date nor the
Gregorain date were correct was in Sweden during a period when they
tried a *gradual* changover. And being out of synch with *everyone* was
so much trouble that they switched back to Julian and stayed that way
until 20 or 30 years later when they finally changed to Gregorian
permanently. 
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