A simple calculator to do basic arithmetic with hours and minutes.
Click on the calculator before you start. This is very important. It'll ignore you
if you don't, because the browser will think that what you're typing is meant for it.
The white rectangle is the one you can type to. If you want to change to the other one,
press enter, right arrow or left arrow. The mouse doesn't make anything happen anymore.
Use the number keys to enter numbers.
Use the + and - keys to add or subtract the top row to the bottom row. This takes a bit
of getting used to, but it's loads easier to program. It's called reverse Polish notation,
by the way, and I haven't just made it up.
If you want to add two times, do first time + second time +
If you want to subtract, do first time + second time -
Use delete or backspace to clear the white rectangle.