Epoch is also acceptable if humans don't need to understand it

I will agree it’s a valid storage but it has to be specified in ms

If you're on Linux exiftool can get the creation date for you: exiftool -p '$CreateDate' -d '%Y-%m-%d' FILENAME, and you could run tgat in a loop over your files, something like:

mkdir -p out
for f in *.jpg
do
createdate=$(exiftool -p '$CreateDate' -d '%Y-%m-%d' "${f}")
cp -p "${f}" "out/${createdate} - ${f}"
done

Obviously don't justbgo running code some stranger just posted on the internet, especially as I haven't tested it, but that should copy images from the current directory to a subdirectory called 'out' with the correct filenames.

I did this in the past and I would search through my notes... If I had notes ffs.

Can you give more context, what are you using? Language / system / etc?