System for producing a system clock signal comprising a local oscillator configured to generate a first clock signal; a temperature detector configured to detect a temperature change above a critical value in an area including the local oscillator; an obtaining module configured to obtain a second clock signal; a clock generator configured to generate the system clock signal in a first manner, using the first clock signal, in a normal mode when the temperature detector does not detect a temperature change above the critical value, and in a second manner different from the first manner, using the second clock signal, in a reverse hold-over mode when the temperature detector has detected a temperature change above the critical value.