timeclock-use-display-time

timeclock-use-display-time is a variable defined in `timeclock.el'.
Its value is

t

Documentation:
If non-nil, use `display-time-hook' for doing mode line updates.
The advantage of this is that one less timer has to be set running
amok in Emacs's process space. The disadvantage is that it requires
you to have `display-time' running. If you don't want to use
`display-time', but still want the mode line to show how much time is
left, set this variable to nil. Changing the value of this variable
while timeclock information is being displayed in the mode line has no
effect. You should call the function `timeclock-mode-line-display' with
a positive argument to force an update.

You can customize this variable.