calculator doubles as stopwatch

Many pocket calculators have a constant factor memory. If a 1 is stored in this memory, depressing the t key adds l to the number already displayed. If the original number displayed is O and the ;!; key is depressed at regular intervals, the number displayed at any given time is equal to the number of times the key has been depressed , which in turn is proportional to time. This means that the calculator can be used as a stopwatch.