Example: Animate text as a timecode display

 With no layers selected in the Timeline panel, double-click the Current Time Format animation preset in the Effects & Presets panel. (You can locate the animation preset by typing its name in the Contains field in the Effects & Presets panel.)

A new text layer is created, with an expression on the Source Text property that makes the text show the current time in the time display format set for the project.

You can use other expressions in the Global category to display time in another format.

To see the expressions on a layer, select the layer and press EE.