I don't know if it's distro specific, but check man crontab. I just figured out how to use this the other day, works great.
Minute Hour Date Month DayOFWeek Application
If any part isn't used replace it with a *
To have ls execute at 755 PM on April 30'th:
55 19 30 apr * ls
And if you don't send it to a log somewhere, you will get an email regarding the execution of "ls"