It is typically a negative term...
clock watcher. noun. an employee who demonstrates lack of interest in a job by watching the time closely to be sure to stop work as soon as the workday or shift is over.

However, I'd beg to differ. I'd argue that so called "clock watchers" actually respect everyone's time. They're attentive enough to come in on time. They do a full day's of work. Some of them do put in extra hours when they can.

As for as leaving...