A method of operation of an electronic device, having a first oscillator and a second oscillator, the method comprising: in a normal mode of operation, counting time based on an output from the first oscillator; and in a low power mode of operation, counting time based on an output from the second oscillator; and further comprising, in the low power mode of operation, repeatedly: calibrating the second oscillator against the first oscillator during a first calibration time period to obtain a first calibration result, recalibrating the second oscillator against the first oscillator during a second calibration time period to obtain a second calibration result, determining a correction factor from the first and second calibration results, subsequently applying the correction factor when counting time based on the output from the second oscillator, recalibrating the second oscillator against the first oscillator during a third calibration time period to obtain a third calibration result, and based on the correction factor and the third calibration result, applying a retrospective correction value to the time that was counted based on the output from the second oscillator during a time period between the second and third calibration periods. in a normal mode of operation, counting time based on an output from the first oscillator; and in a low power mode of operation, counting time based on an output from the second oscillator; and further comprising, in the low power mode of operation, repeatedly: calibrating the second oscillator against the first oscillator during a first calibration time period to obtain a first calibration result, recalibrating the second oscillator against the first oscillator during a second calibration time period to obtain a second calibration result, determining a correction factor from the first and second calibration results, subsequently applying the correction factor when counting time based on the output from the second oscillator, recalibrating the second oscillator against the first oscillator during a third calibration time period to obtain a third calibration result, and based on the correction factor and the third calibration result, applying a retrospective correction value to the time that was counted based on the output from the second oscillator during a time period between the second and third calibration periods. calibrating the second oscillator against the first oscillator during a first calibration time period to obtain a first calibration result, recalibrating the second oscillator against the first oscillator during a second calibration time period to obtain a second calibration result, determining a correction factor from the first and second calibration results, subsequently applying the correction factor when counting time based on the output from the second oscillator, recalibrating the second oscillator against the first oscillator during a third calibration time period to obtain a third calibration result, and based on the correction factor and the third calibration result, applying a retrospective correction value to the time that was counted based on the output from the second oscillator during a time period between the second and third calibration periods.