calc_month.rb

def calc_month(t,n)
  y = t.year
  m = t.month + n
  if m < 1
    y += ((m-1)/12).floor
    m = 12 + (m % -12)
  elsif m > 12
    y += (m/12).floor
    m = ((m-1) % 12) + 1
  end
  d = t.day
  while Time.new(y,m,d).month != m
    d -= 1
  end
  Time.new(y,m,d)
end