Abstract: An electronic watch including a reference isochronous period generator which can be reset or readjusted from outside the watch by a signal representing a standard time period. The generator includes a high frequency source having a period that does not exceed the maximum allowable deviation with respect to the isochronous period supplied by the generator, a dividing counter which receives the high frequency or a submultiple thereof, a detector which detects the signal and then enables the transition of the generator from an autonomous mode in which the isochronal period is provided to a readjustment mode in which the isochronal period is readjusted.