def calc_dt_before_1986(y)
      t = y - 1975
      dt = 45.45      + \
          ( 1.067     + \
          (-1 / 260.0 + \
          (-1 / 718.0)  \
          * t) * t) * t
      return dt
    rescue => e
      raise
    end
def calc_dt_before_1986(y)
      t = y - 1975
      dt = 45.45      + \
          ( 1.067     + \
          (-1 / 260.0 + \
          (-1 / 718.0)  \
          * t) * t) * t
      return dt
    rescue => e
      raise
    end
=========================================================================
 deltaT (1961 <= year && year < 1986)

 @param:  y   (Year coefficient for deltaT calculation)
 @return: dt  (sec)
=========================================================================