timegm(3) implementation for platforms that don't have it (AIX is an example)
Type 'time_t' of the platform can be 32 or 64-bit long, but type 'long long' has to be 64-bit long (or more).
Invalid dates (eg 2021.02.29) return -1, but this can be changed in the future (so don't use this as a validator function).
Supported range for 64-bit time_t: 0-9999 years (might be increased later) mind you, it is proleptic Gregorian (https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar)
Unrelated: aix-mem.c: some other functions that are missing from AIX: mempcpy, memrchr, strlcpy, strlcat, stpncpy, strmode