You can obtain a count of students who have earned a "Diploma" using a Custom Query. This query is in Master Reports | Custom Queries (or browse the extended list and subscribe). This report lists all students where the Diploma Date field on the Reg Form has a date that is within the date range of the report. You can also restrict the results set to just members of a specific Customer Group.
1. This does not look at the Diploma Date found on Credit Programs.
2. This does not require the Diploma Earned checkbox on the Reg Form to be checked; just the date to be within the date range.