It seems that Mac have begun to decrement cron and recommend the use of launchd for job control.
I have found the following links to help me write the job control parameter file.
- man launchd
- a manual page on launchctl
- Mac launchd and launchctl — the OSX alternative to cron a blog by arunxjacob
- Scheduling Timed Jobs at the Mac Developer Library
- Creating a launchd Property List File also at the Mac Developer Library