Cron schedule parser

If you're dependent on cron to run important tasks, you'll want some kind of monitoring. This class is useful for parsing a given cron schedule to see when the cron job should have last run, so you can add monitoring to your application easily by making cron jobs log their events in a database and then writing a monitoring app around this class. Cron schedules can be quite complex to parse since they can involve ranges (n-n) and lists of values or ranges (n,n,n or n,n,n-n,n-n). Finally, days can be specified by calendar days or days of the week. This class supports every possibility at the moment except for weekdays specified in non-numerical format, like Mon, Tues, etc. and */2 syntax for alternation. One possible use for this class would be to write a complete PHP replacement for cron!