The Time & Frequency Research Center is the research department of Shanghai Astronomical Observatory dedicated to the theory and technology of time and frequency and atomic frequency standards. Time and frequency technology and atomic clock technology play an indispensable role in applications including nuclear physics, communications, navigation, surveying and mapping. The Center comprises research groups developing active hydrogen atomic clocks, passive hydrogen atomic clocks, and time and frequency technology. Shanghai Astronomical Observatory is among the earliest institutions in China to conduct time and frequency research. As early as 1884, the Bund Observatory in Shanghai provided time signals by dropping a time ball; in 1914, Xujiahui Observatory began transmitting radio time signals; in 1939, it joined the Bureau International de l’Heure’s international cooperation on Universal Time; in 1964, Shanghai Astronomical Observatory’s Universal Time research reached an advanced international level. From 1958 to 1981, the Observatory broadcast China’s standard time and frequency signals BPV and XSG via shortwave radio. In 1978, it established China’s first local atomic time scale.