How to add a date stamp to a photo I've already taken on an iPhone?

The company I work for requires our photos to be datestamped (not with photoshop), but some of the pictures that were taken by my business partner are not date stamped. Is there any kind of app that can add a date stamp to photos that were already taken on an iPhone?

Look at this Windows software. It works for 30 days free:
http://avpsoft.com/products/timetophoto/