The recently-added tests uncovered problems in the reference implementation on platforms where time_t is an unsigned 32-bit quantity. To reproduce the problem, check out the latest version from Github and run the command 'make check_time_t_alternatives'. On my platform (Ubuntu 13.04 x86-64), this generates about 45,000 lines of output, which I won't bore readers with, but here's one example: -America/New_York Sun Mar 14 06:59:59 2038 UTC = Sun Mar 14 01:59:59 2038 EST isdst=0 -America/New_York Sun Mar 14 07:00:00 2038 UTC = Sun Mar 14 03:00:00 2038 EDT isdst=1 -America/New_York Sun Nov 7 05:59:59 2038 UTC = Sun Nov 7 01:59:59 2038 EDT isdst=1 -America/New_York Sun Nov 7 06:00:00 2038 UTC = Sun Nov 7 01:00:00 2038 EST isdst=0 These transitions should exist with unsigned 32-bit time_t, but they're missing.