Time/Date Stamp questions

[JBrown]

My uncle is a private investigator who has quite an investment in Canon gear. I proposed to him the idea of using a DSLR for optimum video quality + range with his optics. He liked the idea, but came back to me with one problem, he needs a constant date/time stamp shown in the video. Obviously the start of the file is in the metadata, however the people he works for want date/time playing throughout the video.

I know plenty of digital camcorders do this, but do any DSLR's? I have a T3i I was doing some test stuff with, but could not find a way to enable it to be viewed at all time.

Any help is greatly appreciated.