Add date and time to photos or timestamp them

Download and install : FastStone Image Viewer. It is a freeware program.
http://www.faststone.org/FSViewerDetail.htm

Next open the program and select Tools - Batch Convert Selected Images or F3

Next check the  Use Advanced Options and Select the Advanced Options button that appeared.
Select the Text tab - Insert button/dropdown menu - Date Time

You can customize position and font of the text by the red marked buttons.
Next select an output folder and press convert.