The invention concerns an AM/PM display disc (1), rotating in one day about an axis of rotation (2), including two different juxtaposed surfaces (3, 4).It is characterized in that the surfaces are delimited, between a hub (5) and the periphery (6) of said disc (1) by two boundary curves (7, 8), each tangential to said periphery (6) and having a single concavity from said periphery (6) towards said hub (5), the concavities of said curves (7, 8) having the same direction relative to the direction of rotation (A) of said disc (1), and any radial line originating from said axis (2) intersecting, in succession, one of said two surfaces, then the other, and only passing through one of said curves between said hub (5) and said periphery (6).The invention also concerns an AM/PM display device (100) including an aperture (20) and a disc of this type (1), characterized in that said axis (2) is external to said aperture (20) which is symmetrical relative to an axis of symmetry (22) passing through said axis of rotation (2).The invention also concerns a timepiece incorporating a device (100) of this type.