Finally, I removed the Skip Count from the formula (I can't get some reliable results in the last operation) and here is the final version of my automatic dynamic rating, using only 2 user's field :
Rating (auto) AUX =
FormatNumber(math( [Number Plays]*(1 - (( math(FormatDate(Now(),yyyy)*365 + FormatDate(Now(),MM)*31 + FormatDate(Now(),dd)) - math(FormatDate([Last Played,0],yyyy)*365 + FormatDate([Last Played,0],MM)*31 + FormatDate([Last Played,0],dd)) ) / ( math(FormatDate(Now(),yyyy)*365 + FormatDate(Now(),MM)*31 + FormatDate(Now(),dd)) - math(FormatDate([Date Created,0],yyyy)*365 + FormatDate([Date Created,0],MM)*31 + FormatDate([Date Created,0],dd)) +0.001))) ),1)
Rating (auto) =
FormatNumber( If( IsRange(FormatNumber(Math([Rating (auto) AUX]/10),0),0-5) , FormatNumber(Math([Rating (auto) AUX]/10),0) , 5 ) ,0,0,stars,star)
Using this last formula, rating changes at the middle of a decade for the Number Play, ie:
+ 0 to 4 plays, 1 star
+ 5 to 14 plays, 2 stars
+ 15 to 25 plays, 3 stars ...