This is one of those features of Windows Mobile that you have to stumble across by accident to find. This only works for touchscreen devices. If you tap and hold the digital clock in the top bar, you can change it to a little analogue clock, as seen below, which will display in all programs if you have the setting for this turned on.

If your clock isn’t showing all the time and you want it to, go to Start > Settings > System > Clock & Alarms. Go to the third tab that says “More” and check the box that says “Display clock on the title bar in all programs.”