While testing some code I discovered that the tz strftime week numbers are incompatible with the %V week numbers generated by Solaris 2.5.1 strftime and with the GNU C Library strftime. I don't know what the correct definition of week numbers is (and I don't particularly want to know :-) but I thought I should report the problem, since it's two against one. #defining XPG4_1994_04_09 fixes some but not all of the discrepancies. Here are all examples of discrepancies from 1994 through 1998. Each column contains the output of strftime applied to "%U %V %W" for the given date. In the cases marked `*', #defining XPG4_1994_04_09 didn't remove the discrepancy. Solaris date & glibc tz tz with XPG4_1994_04_09 1994-01-01 00 53 00 00 52 00 00 53 00 1994-01-02 01 53 00 01 52 00 01 53 00 1995-01-01 01 53 00 01 52 00 01 53 00 1996-12-30 52 53 53 52 01 53 52 01 53* 1996-12-31 52 53 53 52 01 53 52 01 53* 1997-12-29 52 53 52 52 01 52 52 01 52* 1997-12-30 52 53 52 52 01 52 52 01 52* 1997-12-31 52 53 52 52 01 52 52 01 52* While we're on the subject, perhaps XPG4 behavior should be the default, with non-standard behavior being an option.