#59 timestamps beyond 2036 do not work

[Bernhard M. Wiedemann]

As I found in https://sourceforge.net/p/xindy/bugs/65/
date -u -d @2085978495
2036-02-07T06:28:15 UTC
is the highest timestamp that can be represented as 32-bit unsigned integer counting seconds since 1900-01-01
This is comparable to the year 2038 problem where UNIX' signed int32 time_t overflows after reaching 0x7fffffff.

This affects 64-bit systems as well.