Trying to make a countdown timer(macro) in Open office's Calc?

Okay so i am trying to make a count down timer, that keeps track of the amount of time left that a given amount of time (ala Cooking Timer) I don't need the date, and the time is selected from a table found on the page. (e3) to be exact. I would like it to be in Cell A1 but anywhere would be fine by me