This post explains how to keybind to the same letter both lowercase and uppercase by using vector format. Key detail:
You must use the same syntax for both keybindings!
This always works correctly.
(global-set-key [(control meta ?p)] #'help/insert-datestamp) (global-set-key [(control meta shift ?p)] #'help/insert-timestamp*-no-colons)
This only sometimes works correctly which is the worst kind of working: don’t do it!
(global-set-key (kbd "C-M-p") #'help/insert-datestamp) (global-set-key [(control meta shift ?p)] #'help/insert-timestamp*-no-colons)