Pachyderm is a small utility that quickly checks your system clock to make sure it's sane. This can help workaround the occasional problem where a machine has its clock reset to January 1, 1970 after rebooting.
To use it, simply run it. If Pachyderm thinks your system clock is screwed up, it will ask you to change or confirm it. If the system clock seems OK, Pachyderm will exit without any user action.
One good use for Pachyderm is to put it into your login items so that it runs every time you log in (and, by extension, every time you reboot).
Starting around Mac OS X 10.3 or so, Apple began including some built-in checks in the OS to make sure you weren't starting up with January 1, 1970, which solves the main problem that Pachyderm addresses. However, Pachyderm includes an additional "sanity check" to make sure that the current date is not earlier than the last date Pachyderm ran.