def calc_dt_before_1800(y)
      t = y - 1700
      dt  =  8.83           + \
           ( 0.1603         + \
           (-0.0059285      + \
           ( 0.00013336     + \
           (-1.0 / 1174000.0) \
           * t) * t) * t) * t
      return dt
    rescue => e
      raise
    end
def calc_dt_before_1800(y)
      t = y - 1700
      dt  =  8.83           + \
           ( 0.1603         + \
           (-0.0059285      + \
           ( 0.00013336     + \
           (-1.0 / 1174000.0) \
           * t) * t) * t) * t
      return dt
    rescue => e
      raise
    end
=========================================================================
 deltaT (1700 <= year && year < 1800)

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