display-time-mode

display-time-mode is an interactive autoloaded Lisp function in `time.el'.

It is bound to <menu-bar> <options> <showhide> <showhide-date-time>.

(display-time-mode &optional ARG)

Toggle display of time, load level, and mail flag in mode lines.
With a prefix argument ARG, enable Display Time mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
it if ARG is omitted or nil.

When Display Time mode is enabled, it updates every minute (you
can control the number of seconds between updates by customizing
`display-time-interval'). If `display-time-day-and-date' is
non-nil, the current day and date are displayed as well. This
runs the normal hook `display-time-hook' after each update.