The mcron program represents a complete re-think of the cron concept originally
found in the Berkeley and AT&T unices, and subsequently rationalized by Paul
Vixie. The original idea was to have a daemon that wakes up every minute, scans
a set of files under a special directory, and determines from those files if
any shell commands should be executed in this minute.

The new idea is to read the required command instructions, work out which
command needs to be executed next, and then sleep until the inferred time has
arrived. On waking the commands are run, and the time of the next command is
computed. Furthermore, the specifications are written in scheme, allowing at
the same time simple command execution instructions and very much more flexible
ones to be composed than the original Vixie format. This has several useful
advantages over the original idea.

WWW: http://www.gnu.org/software/mcron/