Using this freeware software you can do it fast way to add time stamp.
Firstly move all photos to one folder then open faststone image viewer software.
Choose the image folder then select all photos after that press the tools tab on the top.
->Tools ->Batch convert selected images -> Advanced options -> Text -> Remove current text and then select -> insert a variable -> File dateandtime ($D1)-> OK ->choose correct format and output destination folder -> Convert
You can find the photos on output folder with time stamped each photos.