def calc_dt_before_1941(y)
      t = y - 1920
      dt  = 21.20     + \
           ( 0.84493  + \
           (-0.076100 + \
           ( 0.0020936) \
           * t) * t) * t
      return dt
    rescue => e
      raise
    end
def calc_dt_before_1941(y)
      t = y - 1920
      dt  = 21.20     + \
           ( 0.84493  + \
           (-0.076100 + \
           ( 0.0020936) \
           * t) * t) * t
      return dt
    rescue => e
      raise
    end
=========================================================================
 deltaT (1920 <= year && year < 1941)

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