Here is a script that will select all images taken in February. Exit darktable before, then:
echo 'delete from selected_images; insert into selected_images (imgid) select id from images where strftime("%m", datetime(datetime_taken/1000000 + unixepoch("0001-01-01 00:00:00"), "unixepoch")) = "02"' | sqlite3 ~/.config/darktable/library.db
This will mark all images taken in February as selected. Start darktable, those images are selected so you can just assigned them a tag if needed (at least temporarily to create a corresponding selection).
Hope this helps !