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-20170118.tar.gz
tmcode-20170118.zip

FTP downloads:
tmcode-20170118.tar.gz
tmcode-20170118.zip

MD5 checksums:
tmcode-20170118.tar.gz: 4d0456243a92b53b4a2f7299e77e7c49
tmcode-20170118.zip: 071293991b5ac7f98e805c656be07f75