Cell formula result unexpected when blank date criteria entered

Hello,

I am hoping someone can help me. Attached is a spreadsheet which I'm hoping will calculate a dollar amount given a certain date criteria. The range of cells I am referring to are located in the spreadsheet:

Worksheet: "variables"
Cells: "O13:Z13"

The formula I am using gives me an answer only when both a start date and end date are entered in L13 and M13 respectively. I would like the formula to work when the end date is either blank or not.

I'm new to the forum and not sure if you have enough information.

Be grateful for any help offered.