How can I create the timer that can automatically stop after a few meters?

Can someone help me to make the timer automatically stop when we move away for a few meters from where the start button is pressed. Everything else is just fine except that I have no idea how to make it stop automatically .

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You could set the starting lat,lon then have a clock timer that tests if the distance exceeds x meters using the great circle formula with the current lat,lon coordinates.

When  that distance is exceeded then stop your timer.

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