cron (hackage): Cron data structure and Attoparsec parser. The idea is to embed it in larger systems which want to roll their own scheduled tasks in a format that people are used to. System.Cron is where all the interesting datatypes live. You will also find scheduleMatches, which you can use to compare a time against a CronSchedule to see if an action needs to be performed.  System.Cron.Parser is where you will find the parsers cronSchedule, crontabEntry and cronTab. To parse individual schedules up to full crontab files.
Keywords: library|mit|parsing|system|text