The objective of the present invention is to propose an accounting method of the consumption of transmitted services per time unit to a decoder in a system implementing a content encrypted by control words, the latter being modified according to a period named crypto-period. This method consists in verifying if the time-current (TC) is comprised in a time variable (Rdate) representative of the authorisation time of use of the service and, if this is the case, decrypting and returning the control words to the decoder, and if it is not the case, debiting an amount (CT) corresponding to a time of use (AT) and recharging the time variable (Rdate) with a corresponding time.