Try making a calculated field like:
COUNTD(IF DATEADD('day', TODAY(), -15)<= [Invoice Date]
THEN [Customer ID]
I re-read what I posted and I apologize for the confusion. I need to calculate clients that re-ordered in 15days or less. It doesnt need to be anchored to today's date
This is how my data looks like
CLIENT ID ORDER DATE
From the above, client 125433F ordered 15days prior 5/25/16 - I want to be able to count this. A sample statement from the report is "In May, 25% of clients orders between 15days"