CronWhen is (a future) gem for retrieving execution time for cron job from crontab string.
Never realy got to implementing it, but I should, probably.
$ gem install cron_when
Or with Bundler in your Gemfile.
gem 'cron_when', :require => false
$ cron_when @hourly
# => 2012-01-27 00:00:00 +0100
Format support is taken from Debian crontab(5) man page