A Clock (Modular) Arithmetic Calculator
Here is a calculator for doing arithmetic (adding, subtracting, and multiplying)
whole numbers on a clock with a given number of hours. If this doesn't
make any sense to you, here's an explanation of what clock
arithmetic is, with an interactive clock that shows the different names
for a number on a clock in another way. Follow this link for another calculator
for renaming numbers on a clock.
Instructions

Choose the operation you want (+, , or x) by holding down the button with
the pulldown menu, and releasing on the one you want.

Type the first number you want to calculate with in the first box.

Type the second number you want to calculate with in the second box.

Type the number of hours on the clock you want to use in the third box
(after "mod").

Click the "=" button to get the smallest nonnegative "name" for the result
of the operation on the clock.
Problems? Got an "E"?

Problems? Got an "E"?

All numbers must be whole numbers. Except for the modulus, they can be
positive, negative, or 0.

The number of hours on the clock (modulus) must be a whole number at least
2.

If you type in a number bigger than the box, it will turn to an "E" (for
Error) because this computer program can't handle numbers above a certain
size.

You have to click "=" or hit the return key after typing for the program
to calculate anything.
Technical Comments

The notation is not quite standard; the "mod m" part should come at the
end of the sentence, not in the middle. And you should use the congruent
symbol (3 horizontal bars) instead of the = sign.

Division is often impossible in modular arithmetic.

This program uses the builtin java mod (%) operator. Because of limitations
on the size of integers in java, this applet can't handle very big numbers.
Original Web Site by Susan Addington