Sorry, I forgot to follow up, but we finally figured it out. It seems that our original formula was trying to create a date from a string. We should have been trying to create a date from a number. The correct formula to create our MMM/YYYY date is:
=date(makedate(num(left(BillDate,4)),num(right(BillDate,2))),'MMM YYYY') where Billdate is a sring of the format, YYYY/MM.