struct tm has fields that were not being set by the implementation,
causing the test to fail when the uninitialized values were compared
with a static initialized result. Zero the structure before filling it.
Closes#17794
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Implement the conversion from UNIX time to broken-down civil time per
the gmtime() and gmtime_r() functions.
Signed-off-by: Peter A. Bigot <pab@pabigot.com>