The SI unit of time has since 1967 been defined by a microwave transition in caesium atoms. Today, caesium atomic clocks constitute the primary time and frequency standards and they are the basis for the Bureau International Poids et Measures (BIPM) coordinated time scale UTC (Universal Time Coordinated). Countries like Denmark, which have not established a national primary standard within this field, obtain traceability from navigation and communication systems.

Within time, we distinguish between time intervals and the time scale.
The SI unit “second” is basic unit for measuring time intervals.

Time intervals are measured typically by using a frequency standard and a counter. The length of a given time interval is determined by counting the number of periods of the frequency standard within the interval.

Establishing a time scale is more complicated as seconds have to be added up continuously. The international atomic time scale (TAI) is based on data from more than 200 atomic clocks in more than 50 national laboratories. It is, however, desirable to have a time scale that is always close to the mean solar time, as determined by Earth’s rotation.
The UTC time scale is therefore obtained by correcting the TAI scale with an integer number of leap seconds so that the UTC time differs less than 0.9 seconds from mean solar time. Leap seconds are added or subtracted when needed, usually at New Year.

As a curiosity it can be noted that Danish legislation does not conform to international practice. The law on time (Law No 83, 29/03/1893) prescribe that the time in Denmark is determined by mean solar time and not the UTC time.

Denmark has no primary or reference laboratories in the field of time and frequency. There are a few accredited calibration laboratories in the field and they obtain traceability through the German DCF-77 radio signal.


Jan Hald

Danish Fundamental Metrology
Matematiktorvet 307
DK-2800 Kongens Lyngby
Phone: +45 45 25 5876
Fax: +45 45 93 1137