Calendar mechanism for a timepiece, includes: at least one mobile (22; 23) for driving an indicator, a wheel (1) which, during normal operation of the timepiece, is driven in a first direction (S1), a lever (16) for actuating the mobile (22; 23), and first and second cams (2, 3) driven by the wheel (1) and controlling the movements of the actuating lever (16), these first and second cams (2, 3) being arranged so that their relative angular position differs depending on the direction of rotation of the wheel (1) so that the movements of the actuating lever (16) cause the mobile (22; 23) to advance periodically during normal operation of the timepiece and do not permit the mobile to advance during backwards time setting of the timepiece causing the wheel to rotate in a second direction (S2) opposite to the first direction (S1).