fenwick (06/02/82)
A normal (non-leap) year is 365 days = 52 weeks + 1 day. Thus if January 1, some non-leap year, occurs on a Monday, then January 1, the next year, occurs on a Tuesday. If January 1, some leap year occurs on a Monday, then January 1, the next year occurs on a Wednesday. Between January 1, 2001, and January 1, 2101, there are 100 years, of which 24 are leap years. Thus if 1/1/2001 is on the n-th day of the week, 1/1/2101 is on the ( (n+124) mod 7 )-th day of the week. Since 124 mod 7 = 5, the first day of each century occurs 5 weekdays (modulo Sunday) later, EXCEPT when we pass over a year that is divisible by 400. In this case, there are 100 normal years and *25* leap years. 125 mod 7 is 6, so from 1/1/2301 to 1/1/2401 we must add 6 weekdays (modulo Sunday). But looking at the series, notice that we add 5 + 5 + 5 + 6 = 21 days every 400 years, so we will cycle through only 4 unique days-of-the-week. We know January 1, 2001 is a Monday (try cal 2001); then 1/1/2101 is a Saturday, 1/1/2201 is a Thursday, 1/1/2301 is a Tuesday, and 1/1/2401 is again a Monday (fortunately). Thus the world will never end. John Eldridge & Robert Snyder harpo!floyd!jce