Toggle display of the amount of time left today in the mode line.
If `timeclock-use-display-time' is non-nil (the default), then
the function `display-time-mode' must be active, and the mode line
will be updated whenever the time display is updated. Otherwise,
the timeclock will use its own sixty second timer to do its
updating. With prefix ARG, turn mode line display on if and only
if ARG is positive. Returns the new status of timeclock mode line
display (non-nil means on).
(fn &optional ARG)