I need to write a formula to display the census count for any day

I have created a Crystal Report to display the total number of patients in care. I have both Start Date and End Date parameters. If I enter both date parameters, I get an accurate count for any day. Now, my problem is that my customer wants the ability to enter 2 different dates (day, week or year) and display the average of the two numbers. I have to run my report 2 times to get 2 different dates. What do I need to do to get the report to display the count for the 2 or more days for which the average is needed? I have Crystal 10

So my data looks like this:

Date Count

01/01/2018 4200

01/22/2018 4800

Average: 4500