Parse-Cron
Parse crontab syntax to determine scheduled run times 
The goal of this gem is to parse a crontab timing specification and determine when the job should be run. It is not a scheduler, it does not run the jobs.
API example
cron_parser = CronParser.new('30 * * * *')
# Next occurrence
next_time = cron_parser.next(Time.now)
# Last occurrence
most_recent_time = cron_parser.last(Time.now)