Continuing my Calendar monologue..
This checkin in YMD.pike:
revision 1.16 date: 2006/01/17 15:58:29; author: mbaehr; state: Exp; lines: +1 -1 fix off by one error
is not a mere "fix" but a compatibility problem (the patch was also incomplete but that's beside the matter). Prior to this change, YMD.year and YMD.years were 0 based, i.e. [0..number_of_years()-1]. With it they become 1 based like all the other similar functions (YMD.day etc). That is imho good, but the compatibility issue is obvious.
I made a simple attempt to make 7.6 compatibility wrappers but it failed, perhaps due to the resolver tricks in the Calendar module. Shall we back out this again until someone manages to get working compat?