Open up Microsoft Notepad (or equivalent) and type:
[date]
2010/09/15
13:29:15
And of course adjust the date and military time accordingly. Then save this file on the root directory of the memory card with the name "TAG.txt"
Turn device on, record a short clip, then turn it off. Now repeat this cycle once more. The second video clip recorded and all videos going forward will have a time and date watermarked on them.