I have never found a truly comprehensive cron tutorial to be comprehensive, it should explain the crontab lines, for example:
2-57/5 * * * * $COMMAND
I have not seen what does it right.
http://www.corntab.com/pages/crontab-gui is a somewhat useful web-based utility for creating crontab. It does not explain the concept, but allows you to build the crontab line by choosing the time you need without having to remember the syntax. Unfortunately, this is not possible with more complex examples, but, nevertheless, is a useful tool.
http://cronwtf.github.com/ is a somewhat useful web-based utility for parsing crontab lines and explaining English when the team will work. This does not apply to my example above.
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