def calc_dt_before_1920(y)
      t = y - 1900
      dt  = -2.79      + \
           ( 1.494119  + \
           (-0.0598939 + \
           ( 0.0061966 + \
           (-0.000197  ) \
           * t) * t) * t) * t
      return dt
    rescue => e
      raise
    end
def calc_dt_before_1920(y)
      t = y - 1900
      dt  = -2.79      + \
           ( 1.494119  + \
           (-0.0598939 + \
           ( 0.0061966 + \
           (-0.000197  ) \
           * t) * t) * t) * t
      return dt
    rescue => e
      raise
    end
=========================================================================
 deltaT (1900 <= year && year < 1920)

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