# File lib/date.rb, line 963
  def >> (n)
    y, m = clfloor(year * 12 + (mon - 1) + n, 12)
    m,   = clfloor(m + 1, 1)
    d = mday
    d -= 1 until jd2 = self.class.valid_civil?(y, m, d, ns?)
    self + (jd2 - jd)
  end