You'll find the countdown timer located within the "Misc" group of components in the Builder:

To create a dynamic/evergreen countdown, choose the "Evergeen" value for the "COUNTDOWN TYPE" option in the countdown settings area.

Then, set the number of minutes you want the timer to start counting from.

If you want to set a bigger delay, for example : 2 days and 4 hours, just give its equivalent in minutes which is 3120 minutes (2 * 24 * 60 )+(4 * 60).

The second option of the evergreen countdown is the "RE-VISIT ACTION", determines what will happen if the same visitor is back again to your page / popup is displayed to them again :

Reset Countdown : The countdown will restart again.
Continue : The countdown will continue normally without restarting from the beginning.

To create a countdown with a fixed end time e.g. Sunday at midnight, choose the "Normal" countdown option and simply set the end date/time in the little calendar popup (to set the precise end time click the clock icon).

Please note, the time zone that the end time syncs with is determined by your account's Workspace settings. You can access these via the left pull-out menu on your account dashboard:

How to create evergreen/dynamic countdown timer?