Display time since post was published

Hi all Im in the middle of designing a new theme, I know how to display the time and date a post was published, but I would like to do something different I would like to display how long ago something was published in minutes so that all posts published under 60 minutes are shown in minutes, every thing between 1 hour and 24 hours is shows a been published X hours ago and every thing published over 24 hours is shows as X days old.

I see that CNN politics blog does something very similar and was wondering how this can be done in WordPress, any help would be very appreciated.