time signal
A time signal is a service that disseminates accurate time to the public. In Japan, NTT's telephone time service (dial 117) is the best-known example, announcing the time based on NICT's Japanese Standard Time with a long tone at each exact hour. Internationally, services like WWV (USA), MSF (UK), and DCF77 (Germany) broadcast time signals via radio.
Telephone speaking clocks provide the time as a voice announcement on a dedicated phone number. Radio time signals, such as the BBC pips or NHK's hourly tones, broadcast short beeps followed by a longer tone at the top of the hour. Standard-frequency stations like JJY in Japan deliver coded time data to radio-controlled clocks. Digital-era alternatives include NTP (Network Time Protocol), which synchronizes computer clocks over the internet to within milliseconds.
Telephone time signals are limited by call routing latency (tens to hundreds of milliseconds) and do not guarantee sub-second accuracy. Radio broadcasts travel at the speed of light, introducing negligible propagation delay (about 0.3 ms per 100 km), though receiver processing adds further latency. Digital terrestrial TV carries a roughly 2-second encoding delay, making it unsuitable for precise time signals. For everyday clock-setting, radio-controlled clocks receiving standard-frequency transmissions or NTP-synchronized devices offer the best practical accuracy.
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nict
NICT is Japan's national research institute responsible for generating, maintaining, and distributing Japan Standard Time (JST), operating a fleet of atomic clocks and transmitting the JJY time signal.
jjy
JJY is the call sign of Japan's long-wave standard-frequency time signal, operated by NICT from two transmitters - Otakadoya-yama (40 kHz) in Fukushima and Hagane-yama (60 kHz) in Saga - broadcasting JST around the clock.
jst
JST (Japan Standard Time) is the standard time of Japan at UTC+9, based on the 135th meridian east. Japan does not observe daylight saving time.
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