Avoid Using POSIX time_t for telling time

This is John Sauter's code for performing leapsecond-aware date and time arithmetic on struct tm. John didn't have a good place to host it, so I am hosting it here for him.

The code is available in two formats, .tar.gz and .zip, and via two protocols, HTTP and anonymous FTP.

HTTP downloads:
tmcode-20170110.tar.gz
tmcode-20170110.zip

FTP downloads:
tmcode-20170110.tar.gz
tmcode-20170110.zip

MD5 checksums:
tmcode-20170110.tar.gz: 613f1f7e8955c0045e4be9f2ac63c241
tmcode-20170110.zip: 78ae617cca5b202c1bcd08b5f3797c3b