Timestamp is a program that lets you calculate the exact amount of time you dedicate to a particular task (mainly working, for obvious reasons).
The way it works is really simple. When you're ready to work click on the Punch IN button. When you stop working click on the same button, whose name will have changed to Punch OUT.
As you repeat this action every day the program creates a work time log that you can export to Excel or as a plain text file.
The program also has other options like including lunch breaks (which you can customize), activating a notification to tell you when you've reached the day's maximum number of hours worked, or using automated features like starting the Punch IN at Windows startup or notifying you when Punch OUT has been left on.
Requires Android 2.3 or higher