def calc_dt_before_1700(y)
      t = y - 1600
      dt  = 120           + \
           ( -0.9808      + \
           ( -0.01532     + \
           (  1.0 / 7129.0) \
           * t) * t) * t
      return dt
    rescue => e
      raise
    end
def calc_dt_before_1700(y)
      t = y - 1600
      dt  = 120           + \
           ( -0.9808      + \
           ( -0.01532     + \
           (  1.0 / 7129.0) \
           * t) * t) * t
      return dt
    rescue => e
      raise
    end
=========================================================================
 deltaT (1600 <= year && year < 1700)

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