The invention relates to a method for watermarking at least one timestamp in a set of support data, comprising the following steps for each timestamp: association with the timestamp of the value at a given time of a reference timestamp signal, that is a determinist signal varying over time in a given temporal reference and being written as s(t), wherein t is incremented according to an incrementing step equal to one predetermined time unit; and watermarking of the value in the set of support data. The invention also relates to a corresponding decoding method comprising the following steps: reading, in the set of support data, of at least one value of a phase-shifted timestamp signal, of the same form as the reference timestamp signal but having in relation to this signal an unknown phase shift φ in a given temporal reference, the previous watermarking of each timestamp consisting in a watermarking of the value at a given watermarking time of the reference timestamp signal; estimation of the phase shift φ from the at least one read value of the phase-shifted timestamp signal and from the knowledge of the reference timestamp signal; and for the at least one read value, read at a given time tr, determination of an associated timestamp h associated according to the read time tr and the estimated phase shift φ.