Timestamp as a watermark on an iOS video

Question

I want to add a Timestamp watermark to my already recorded and exciting videos in the iOS app.

Here I have an already asked question and answer on stackoverflow.com

How to add watermark to captured video on iOS

and some other examples on the Internet.

But this is for a still watermark. What I'm trying to achieve is

if the user started recording video on December 20, 2011 at 17:32:24, and the video was 3 minutes, the timestamp at the beginning of the video will read 12/20/2011 // 17: 32: 24. The time will change as the video warms up and the time stamp on the last frame of the video will be read 12/20/2011 // 17: 35: 24.

I am confused that how can I do this? I tried a lot, but did not try.

Any recommendations highly appreciated.

thanks