Show the number of days between the customer record's creation and it's date of first sale

1. Create a Saved Customer Search, List > Search > Saved Searches > New.
2. Select Customer
3. Navigate to the Results tab and add the field Formula (Text)
4. Set the formula to:

ROUND({firstsaledate} - {datecreated})

5. Change the Custom Label and click Save.

Note: You may also use the same formula to count the difference of any 2 date fields. You'll just need to replace {firstsaledate} and {datecreated}.