An electronic watch or other timepiece has a clockwork driven by an electronic oscillator having a quartz crystal. A variable trimmer condenser is connected in parallel with this crystal for adjusting the operating frequency of the oscillator and hence the accuracy of the watch. A counter-electrode or armature closely spaced from a fixed plate of this trimmer condenser is carried on or forms part of a bimetallic strip which normally lies against an abutment in the condenser housing so that only when the ambient temperature passes a predetermined threshold does the strip pull away from the abutment and later the capacitance of the condenser, thereby changing the operating frequency of the oscillator in a manner calculated to compensate for the effects of temperature variations on the crystal and to maintain that frequency stable within narrow limits. The strip can be arranged to make such compensation above or below the threshold whose value can be adjusted with the aid of the coacting setscrew.