You can use the rufus-scheduler gem to parse cronline/crontab/cron with Ruby.

Gemfile:

gem 'rufus-scheduler'

Example:

require 'rufus/scheduler/cronline'
cron = Rufus::Scheduler::CronLine.new("00 00 1 * *") # 00:00 every first day of the month
#
next_time = cron.next_time(Time.current)
previous_time = cron.previous_time(Time.current)

rufus-scheduler's cronline implementation: https://github.com/jmettraux/rufus-scheduler/blob/master/lib/rufus/scheduler/cronline.rb

See tests for details and syntax: https://github.com/jmettraux/rufus-scheduler/blob/master/spec/cronline_spec.rb

Cron configuration examples.