So you've got running-total in your tags - what did you try?
I would create a temporary table and generate the running total, and
derive the month using the 'Month' function, and also derive the day of the
month. If you sort by month and day each month's data will give you a
running total that is what you want. This means you go month-by-month, and
calculate the temp table for all salespersons.
It's not super elegant, but it'll give you what you want.