How does cron internally schedule jobs

Question

How do "modern" cron daemons internally schedule their jobs? Some cronds used to schedule a run every so often via at. So after a crontab is written out, does crond:

1. Parse the crontab for all future events and the sleep for the intervals?
2. Poll an aggregated crontab database every minute to determine if the current time matches the schedule pattern?
3. Other?

Thanks,