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. Program logic circuitry for the dividing counter causes the counter to proceed to a counting cycle of a suitable number of bits when the counter is in the autonomous mode and to predetermine this proper number of bits on the basis of the standard time period when the counter is in the readjustment mode, the logic circuitry including a memory for storing the count obtained by the counter at the end of the standard time period, the counter being reset to zero at the beginning of this period.
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