Running command at certain time or after that as soon as possible
Solution 1:
The answer to your problem is Anacron. It comes with Ubuntu by default. Just put your script in one of these folders and make it executable:
/etc/cron.hourly # run hourly, only available since Ubuntu 14.04
/etc/cron.daily # run daily
/etc/cron.weekly # run weekly
/etc/cron.monthly # run monthly
For finetuning check out the anacron manpage. The difference between anacron and crontab is that anacron jobs will run later if they can't run at the specified time.