The problems at Japan's Fukushima-1 nuclear plant have had an unexpected impact on the country's ability to keep time: a transmitter that sends the national time signal to many thousands of clocks and watches has been forced offline making the timepieces a little less reliable than usual.

The transmitter is on top of Mount Otakadoya in Fukushima prefecture, about 790 meters above sea level and 16 kilometers from the stricken Fukushima-1 power station. The radio station, called JJY, sends a signal that reaches most of Japan including the capital, Tokyo.

The site is well within a government-mandated 20-kilometer evacuation zone and engineers were forced to abandon it on the evening of March 12 (at 7:46 p.m. JST to be precise -- they were keeping time).

As they left, they powered down the transmitter and that left some devices without a reference time signal.

The devices, which include things like watches, traffic signals, and taxi meters, have internal clocks that keep running in the absence of the radio signal, but the ability of some to auto correct has been lost.

A second transmitter exists in western Japan on the island of Kyushu. Its signal also covers most of Japan, but it transmits on a different frequency. Some devices don't have a tuner for the second frequency while others might be hampered by the signal, which is weaker in Tokyo and east Japan.

The National Institute of Information and Communications and Technology that runs the transmitter sites said it doesn't know when JJY service from Fukushima will resume.

The evacuation zone around the Fukushima-1 plant remains in place as work continues to bring the plant under control. Airborne radiation levels have been steadily dropping over the last few days, but the situation at the plant remains serious.

It could be some time before engineers are allowed back to the station and JJY returns to the air.

Japan is not alone in broadcasting national time over radio.

The U.S. national time is delivered via WWV in Fort Collins, Colorado, and WWVH in Kekaha, Hawaii; and dedicated radio services also exist in Argentina, Belarus, Brazil, Canada, China, Ecuador, Finland, Germany, South Korea, Kyrgyzstan, Russia, Spain and Venezuela. In France and the U.K., the time signal is encoded alongside longwave radio broadcast stations.

Martyn Williams covers Japan and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is martyn_williams@idg.com