date.c: abort if the system time cannot handle one of our timestamps

date.c: abort if the system time cannot handle one of our timestamps

We are about to switch to a new data type for time stamps that is
definitely not smaller or equal, but larger or equal to time_t.

So before using the system functions to process or format timestamps,
let's make extra certain that they can handle what we feed them.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>