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Your numbers are saved on this device, in your own browser, so they are still here when you come back. We work them out on our server and never write them to our disk. There is no account unless you choose to make one. Clearing your browser data clears them.
Your numbers apply as soon as you type them, and they are saved in this browser so they are here next time. Clearing your browser data clears them.
Sign in for one reason: to keep your record when you use another device.