Date2name is an Emacs package inspired by date2name by novoid. It can add ISO-Timestamps to files in dired.
Install date2name from melpa.
In dired call date2name-dired-add-date-to-name to add the modification date of the file to the filename. With the prefix C-u the user is instead prompted to insert a date using the wonderful org-read-date. With two prefixes C-u the user is only prompted once for all marked files.
The same is true for filetags-dired-add-datetime-to-name only that it also adds the time.
The format is again the same as in date2name of novoid.
2018-07-06_test.txt 2018-07-06T13.53.09_test.txt
Unfortunately right now this format should not be changed as it must match a corresponding regexp to remove potential timestamps already in the filename.
Also inspired by novoid’s work: filetags.el