Year To Date Formula

Is there a simple formula for calculating a shifting YTD point?

I have a data table where by A3:A14 is my date range (month) and B3:14) is my data. I also have a drop down box in A18 (months) which allows the user to choose a month to select the YTD figure up to. So, for example, if we're in June and the user uses the drop down to select May I need the WTD to reflect the YTD figures as it was in May etc.