Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 306

AFTER EFFECTS CS3 300
User Guide
A new text layer is created, with an expression on the Source Text property that makes the text show the current time
in the project's current time display format.
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.
See also
"Global objects" on page 556