Jean:

When the year End process is run, it sets the YTD figures to zero for each Vendor. Payables doesn't really know about dates for those PTD and YTD figures. It just accumulates dollars and quantities until the next time that the Year-End process is run which sets them to zero again.

If you have the Adagio Toolkit (add-on product from Dakota Software) it does have a function to recalculate the PTD and YTD figures.