There are two ways to do this, using either the unix time stamp or the mysql time stamp.
Add an extra field to your news table, so that when you enter a new story you also enter the time when the item was entered.
Then you get the current time when the user displays that page, subtract from it the time in the database table, and you have the difference in time.
You can leave this in seconds, or you can play around with it to give hours and minutes as well.
HTH - Blu