Accounts that have access to the countdown feature are able to create a special custom field type from the manage fields page.

When you choose to create a new countdown field you are able to set the end date, end time, timezone, prefix, suffix, and final day message in order to customize exactly what the customer will see as time progresses. The prefix, days left, and suffix will be used as text for the length of the countdown until the final day in which the last 24 hours message overrides the previous text.

You're then able to simply drag and drop the new custom countdown element on to the canvas when designing in the ad builder. Once published the design will update automatically as the end date draws closer.