GNU mcron (Mellor's cron) is a 100% compatible replacement for Vixie cron.
It is written in pure Guile, and allows configuration files to be written in scheme (as well as Vixie's original format) for infinite flexibility in specifying when jobs should be run.
Mcron was written by Dale Mellor (see his personal web pages). Mcron was proposed to the FSF (2003-07-11) as a component of the GNU system, and accepted on 2003-12-02. Version 1.0.1 was the first release of mcron as a GNU package homed at gnu.org.