No values available in Applies to Month with Days dropdown in Partial Month Determination dialogue

Summary:

No values available in Applies to Month with Days dropdown in Partial Month Determination Proration Detail dialogue

Content (required):

My client has a requirement to prorate the rate for one plan type based on coverage start date when coverage begins mid-month. This calculation is based on the exact number of calendar days of coverage with the denominator being the number of days in the coverage begin month.

We are trying a Proration Formula, but I would also like to test the option of applying the percentage based on the day of month so we can compare the results.

My issue is that there is nothing available in the Applies to Month with Days dropdown. I have been hunting for a lookup or other configuration item to populate this dropdown.