This is v3 of this patch set incorporating review comments on v2. I've minimized the size of the localtime.c.patch file by beefing up #defines in the wrapper localtime.cc. I believe I've addressed all comments.
This patch set has been tested on both 64- and 32-bit Cygwin. The initial shell opens with timezone correctly set. xclock, date, uptime all show the correct local time. Overriding TZ in the environment on a call to date, as in 'TZ=Asia/Tokyo date' shows correctly adjusted time. ..mark [PATCH 1/3 v3] Cygwin: tzcode resync: basics [PATCH 2/3 v3] Cygwin: tzcode resync: imports [PATCH 3/3 v3] Cygwin: tzcode resync: details