I'd buy an older PDA to keep on site and find a programmer on rentacoder.com or one of the other freelance programming sites and have a program written. It should be very simple and inexpensive. Keep in mind though with a timeclock if your start time is 8:00am and someone shows up at 7:53 each day he will pick up 7 minutes each day which is fine if he is working but if he is hanging around waiting for his coworkers, stops by the coffee pot, etc you will be paying for nothing.
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