How to modify time function to read “X hours or days”

I need to modify the time stamp for each post to say “Posted 3 hours Ago” instead of “Posted April 4, 2010 4:30 PM” and when a post gets to be more than 24 hours then say “X days ago” until it gets to a week … after a week then revert to normal date structure.

I have done a lot of searching and haven’t found anything.

Any help / guidance would be greatly appreciated and probably help the community for others who want this functionality … in-fact, this could be a cool thing to build into core?!?