mktime() returns a time_t. time_t is often 32 bits, even on many architectures

that feature 64 bit 'long'.

Some systems have 64 bit time_t and deal with years beyond 2038. However, even
some of the systems with 64 bit time_t returns -1 for dates beyond 03:14:07
UTC, January 19, 2038. (Such as AIX 5100-06)