Lenght of membership

[TrippelA]

I need some help with a DAX formula to calculate the length of membership.

I have used the following formula

Active Clients = CALCULATE ([Total Clients]; FILTERS (Clients1; Clients1 [Client: Date Contract Start] <= LASTDATE ('Date uses' [Date]) && (Clients1 [Client: Date Contract End]> = FIRSTDATE (' Date User '[date])))

The problem is that it does not include they who is still a member because there is no date registered in the "Date Contract End" . I expect that I must use IF here in some context, so that if the Date Contrat End is empty then TODAY  is selected.

Hope some one can help me with this.