Surely the date difference between two dates which are the same IS zero.
There are no dates different. What would two dates 1 day apart show? 2? Not
sure I follow you

Logically, yes. But not in my case, because I need this formula to deternime
how long a person has been a client. If they came in only once, and never
again, they were our client for one day. Not zero days. You see?