For what it's worth, it seems you don't really need an YTD analysis rather than a moving annual total or twelve months rolling date instead. YTD means (speaking with today's date) you want to analyze from "Ene 1 2009" to "Mar 8 2009" compared to "Ene 1 2010" to "Mar 8 2010" (today).
I'd use
AddMonths(DateField, -12)
in your script and expressions so you can select the last 12 months (i.e: from today's date minus one year to today's date or selected 01/03/2009, sum sales from 01/03/2008 to 01/03/2009). Your expression then should look like
Sum({< Date = {">=$(=AddMonths(Date, -12))<=$(VMax)"} >} Sales)