In the Export section, twirl down the Effect panel. There you'll find a checkbox for Timecode overlay.
Alternatively, in the edit section, you can create a Timecode effect on either an Adjustment layer that spans the entirety of the sequence, or a Transparent video layer to which you apply the same effect. An adjustment layer will feature a 01:00:00:00 format. If you find that annoying, you can use a Transparent video layer: it will show 00:00:00:00 as a starting point.