If you click the button, it will automatically fill the time with the current time. Or else, you can type the time in manually.
ps. i know there are timeclock type apps around, but dont know if there are any free ones to do what you want; since your needs seem simple enough this seems like a reasonable task.
though it sounds like something you might want to do as a web app (php, ruby).
this sounds like a good assignment for me to post in the programming school actually.
maybe you can elaborate a little more about your needs - like how much do you want implemented to prevent people from putting in fake data, clocking in for each other, etc.