DateComp not working - Trying to Update a Formula Field

DateComp not working - Trying to Update a Formula Field

I'm using the following in a Formula field, but the value does not update from 0.

Datecomp(Now(),${Deals.Date Ready to Invoice})

I'm trying to keep an updated record of how many days since the Deal Stage changed to "Ready to Invoice".  Is there another way if the above formula won't update?