Most people I know who work full-time jobs get to work early and leave late more often than not. November 24 is Go Home On Time Day, which is the ideal reason to actually finish when you're supposed to finish.
The Go Home On Time Day site will send you a reminder to go home if you sign up, and includes the facility to print your own "leave pass". Though obviously intended as a tongue-in-cheek gesture, it's a sad reflection of our addiction to overwork that we might even briefly feel we need special permission simply to work regular hours.
We're mentioning this a day early because finishing on time generally requires both commitment and planning. In an ideal world, working outside your normal hours would be the exception, not the rule. However, even one day doing that is a step towards the goal of doing it more often.