I have the following functions defined in my ~/.emacs/personal/custom.el
file (I use prelude):
(defun timestamp ()
(interactive)
(insert (format-time-string "%Y-%m-%dT%H:%M:%S%:z")))
(defun today ()
(interactive)
(insert (format-time-string "%Y-%m-%d")))
I use them with M-x
and then use the helm popup to find them.
Really convenient to change the date or time in the front matter of a hugo post for instance