If you want to get out from office after the finish time immediately, use this app.
Finish Timer for iPhone 3GS, iPhone 4, iPhone 4S, iPod touch (3rd generation), iPod touch (4th generation) and iPad on the iTunes App Store
This application will notify remain time till the finish time after setting start time and finish time. Even if the app go background, notification will be working.
You can see notifications as follows.
Launching the app, you can see the remain time till the finish time.
You can configure the start time and the finish time, and alarm on/off.
Please enjoy your after hours!