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National Institute of Information and Communications Technology (NICT) generates and supplies Japan Standard Time (JST). JST is made from an average atomic time which is calculated by using 18 commercial Cs atomic clocks. In this calculation, there are some original methods in estimating the clock rate and clock weighting. The actual signal of JST is generated by the realization of this average atomic time. In this process, the frequency control method was optimized recently and frequency stability of JST has been largely improved.