International Atomic Time (TAI) is a global time scale maintained by the International Bureau of Weights and Measures (BIPM) in France. The Coordinated Universal Time (UTC), which is the primary time scale used by international communication, navigation, research, and trade, is directly derived from it by adding a certain number of leap seconds (37 as of today) to TAI.

TAI is formed from the weighted average of more than 400 atomic clocks placed at national metrology institutes and research institutions world-wide. Most of them maintain many atomic clocks while few run only a single clock.

The institutions continuously compare their local representation of UTC – labelled UTC(k), where k designates the institution, with each other, and at the end of each month, BIPM combines these measurements to calculate the weighted average that forms the most stable time scale possible. This combined time scale is published monthly by the BIPM Time Department as “Circular T” along with the differences between UTC and the individual UTC(k) representations (

In the fall of 2022, DFM established the first Danish representation of UTC labelled UTC(DFM) and from March 26th 2023 on, as one of the last countries, the legal time in Denmark was changed to follow UTC.

Since October 2022, DFM has been reporting UTC(DFM) to BIPM as one of the worldwide groups. Even though we only maintain a single atomic clock and have only participated in little more than a year, UTC(DFM) is now weighted with 1.243 %. This may seem like a low number but, since the maximum weight for a single atomic clock is 1.644 %, we are actually very close to the maximum. At DFM, we are very proud of this achievement, as it is the highest among all institutes running only a single clock. Since the weights are assigned by BIPM based on the clocks’ contribution to TAI overall stability, we take this as a clear sign that our clock performs excellently.

The work is supported by funds from Danish Agency for Higher Education and Science.

DFM is one of the seven Approved Technological Service (in short “GTS” in Danish) institutes in Denmark.