How do I add a date stamp to a photo after it's already been downloaded to a file?

If you wish to add a date stamp to the face of the
photos...the following free software may be worth
a try.

FWIW...you can add the date stamp to one
or a batch of photos using the freeware
FastStone Image Viewer.

(FWIW...it's always a good idea to create a System
Restore point before installing software or updates)

FastStone Image Viewer
http://www.faststone.org/FSViewerDetail.htm

I suggest that you create a new folder and add
copies of your photos for experimental purposes.
If you are unhappy with the result your originals
will be untouched.

"Basic steps to add dates to image files
using Fast Stone Image Viewer"..
(FWIW...this is easier to do than it is to explain)

(You'll have to experiment with position, font
size, color and the various Variables to achieve
the result you desire.)

Tools / Open Batch Convert / Batch Convert tab...

Check the box...Use advanced options...

Advanced Options button / Text tab /
Check the box...Add Text...
(If the white field has any text in it...
clear it by selecting and press your Delete key...

Open the drop window..."Insert a Variable"
choose...EXIF DateTime / Date and Time...OK...

Browse to the files you wish to append...select
them and left click the Add button...

Choose an Output Format...

Choose an Output folder...

Check the 'Rename' box...

Click the Convert button...

When the files are converted..click Done...

Now browse to your save folder and see the result...