# returns the number of months between date_a and date_b,
# or -1 if date_a < date_b
def mon_diff(date_a, date_b)
  if date_a < date_b
    mon = date_b.mon - date_a.mon + 1
    mon += 12 * (date_b.year - date_a.year)
  else
    mon = nil
  end
  mon
end