The hourmeter may gain time if the key is left in the ON position even if the unit is not running. For example you may have run out of fuel and forgot to turn the key off while you went after more fuel, or while getting off of the machine after using it you may have accidentally hit the key and turned it to the ON position. We suggest you remove the key when you are finished with the mower.
The hourmeter, like a car odomometer, can not be reset or turned back. The only options are to make notes of what the correct hour(s) should be or to replace the hourmeter, this is not a warranty failure.