Count Down

We need a javascript coundown script that will allow us to set a weekly fundraising goal. Starting on Sunday of the week the counter will show the complete goal for the week, every 15 minutes it will countdown dollars in real-time until Friday when the counter will reach zero dollars left. On Friday and Saturday instead of counting down the dollars the counter will add dollars to the weekly goal showing that we have exceeded our goal. Ideally we would only want to update one variable, the total weekly goal. The script should calculate the rest of the information automatically based ont the weekly goal.