The countdown timer in your header area (and the Livestream layout) will be tied to any events that have the "Live Service" category assigned. To manage this, go to Events > Events, and select the "Add a New Event" button (or open an existing one in the list).

Within the event settings, the "start date/time" will determine when the timer stops and your event is "Live". The "end date/time" will signify the end of the "Live" event, and if you have a recurring event in place, the timer will start again.

Your event must have the "Live Service" category assigned, in order to control the timer.