OK....If you want formulas to list the unique Cust #'s and the count of unique dates for each, try this:
A2:A4000 containing Cust #'s
B2:B4000 containing dates
This ARRAY FORMULA, committed by holding down CTRL and Shift when you press ENTER
(instead of just pressing ENTER) returns the first Cust # value:
...and this ARRAY FORMULA begins listing subsequent Cust # values:
Copy F3 and paste the formula into F4 and down as far as you need.
This REGULAR FORMULA returns the count of unique dates for each Cust #:
Copy that formula down as far as you need.
Does that help?