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A cron-like program that doesn't go by time: anacron (like "anac(h)ronistic") is a periodic command scheduler. It executes commands at intervals specified in days. Unlike cron, it does not assume that the system is running continuously. It can therefore be used to control the execution of daily, weekly and monthly jobs (or anything with a period of n days), on systems that don't run 24 hours a day. When installed and configured properly, anacron will make sure that the commands are run at the specified intervals as closely as machine-uptime permits.
hc-cron is a modified version of Paul Vixie's widely used cron daemon. Like the original program it runs specified jobs at periodic intervals. However, the original crond relies on the computer running continuously, otherwise jobs will be missed. This problem is addressed by hc-cron, that is indended for use on home-computers that are typically turned off several times a day; hc-cron will remember the time when it was shut down and catch up jobs that have occurred during down time when it is started again.