Something that I've never wanted to do, but would imagine it would be quite simple to just create an Action.
A very simple way would be to download Exifer click here which is freeware.
In Exifer with a folder of images open go to "Image">>"Watermark" A box will open and over on the righthand side select "Others". from the dropdown box select Exif Image and then "Date Taken". Click continue, here you will be able to select a folder to save the time stamped images to.