exact-time.eu

From atomic clock to your screen

The time on this website starts in the atomic clocks of Germany’s national metrology institute and reaches you in four steps. Here is what happens at each step, and how much accuracy each one costs.

  1. 1

    Atomic clocks worldwide

    More than 400 clocks in about 80 laboratories are compared to form Coordinated Universal Time (UTC).

    a few nanoseconds
  2. 2

    PTB, Braunschweig

    Germany’s national time UTC(PTB), served by its public time servers.

    well under a millisecond
  3. 3

    Our reference clock, Frankfurt

    Follows PTB continuously, cross-checked with four other institutes.

    about 1 ms
  4. 4

    Your browser

    Measures its own network delay and corrects for it.

    typically ±10–40 ms

What an atomic clock is

Since 1967, the second has been defined by nature rather than by the Earth’s rotation: it is the time it takes for 9,192,631,770 oscillations of the radiation from a particular transition in the caesium-133 atom. An atomic clock counts these oscillations. The most accurate ones, called caesium fountains, cool clouds of caesium atoms with lasers, toss them upwards and measure them as they fall; they would gain or lose less than a second in tens of millions of years. Laboratories also run hydrogen masers, which are exceptionally stable from day to day, and commercial caesium clocks.

Are all atomic clocks synchronised?

No, and they do not need to be. Every clock drifts very slightly in its own way, so instead of forcing them to agree, laboratories compare them. Clocks on different continents are compared through satellites (GPS and other navigation systems, and dedicated two-way satellite links) and increasingly through optical fibres.

The International Bureau of Weights and Measures (BIPM) near Paris collects these comparisons from more than 400 atomic clocks in about 80 laboratories. It combines them into a weighted average, keeps its rate correct with the best primary standards, and the result is International Atomic Time (TAI). UTC, the basis of civil time everywhere, is TAI adjusted by leap seconds to stay close to the Earth’s rotation: since 2017, UTC has been exactly 37 seconds behind TAI.

There is a catch: UTC is calculated afterwards. The BIPM publishes it once a month in a bulletin called Circular T, which lists how far each laboratory’s clock was from UTC on each day. So in real time, nobody can read UTC directly. Instead, each national laboratory keeps its own real-time version, called UTC(k): in Germany UTC(PTB), in the UK UTC(NPL), in Italy UTC(IT). They steer it using Circular T, so the leading laboratories stay within a few nanoseconds (billionths of a second) of UTC.

Step 2: from PTB to the internet

PTB in Braunschweig is Germany’s national metrology institute and one of the leading laboratories in the world: it builds caesium fountains, contributes heavily to TAI, and its time is Germany’s legal time. PTB also runs public time servers (ptbtime1 to ptbtime4.ptb.de) connected directly to UTC(PTB). They answer requests using the Network Time Protocol (NTP), the standard way computers synchronise their clocks over the internet.

Step 3: our reference clock

Our reference clock is a dedicated server in Frankfurt running chrony, the synchronisation software used in laboratories and telecom networks. It asks PTB for the time many times an hour. For each exchange it records four moments (when the request left, when PTB received it, when PTB replied, and when the reply arrived), which lets it separate the network delay from the difference between the clocks. The round trip to PTB takes only about 6 milliseconds, which leaves little room for error.

chrony also asks the institutes of Italy (INRIM), Poland (GUM), the United Kingdom (NPL) and Sweden (Netnod). If PTB ever disagreed with them, chrony would notice and set it aside. It then adjusts our server’s clock gently and continuously, so it normally stays within about a millisecond of UTC(PTB). The bound shown above is calculated live from chrony’s own figures and is deliberately conservative.

Step 4: from our server to your screen

Browsers cannot use NTP, so your browser does the same calculation over ordinary web requests. When you open the clock, it asks our reference clock for the time eight times, measures each round trip, and keeps the fastest ones. Half of the shortest round trip is the most the time could be off, because nobody can tell whether the request or the reply took longer. Add the reference clock’s own uncertainty and one screen refresh (8 to 16 ms), and you get the accuracy shown on the clock page.

On a typical connection this is ±10 to ±40 milliseconds: far better than any wristwatch needs, and about ten times faster than a human can react. To do better, you would need a GPS receiver or a dedicated network protocol such as PTP, which is what laboratories, telecoms and stock exchanges use.

And radio-controlled watches?

The German time signal DCF77 is controlled by PTB too: its time comes from PTB’s atomic clocks and is broadcast on long wave from Mainflingen near Frankfurt. Radio waves travel about 300 km per millisecond, so a watch 1,500 km away receives the time 5 milliseconds late. When you use our Watch sync page, the same chain applies: the signal’s seconds are timed by our reference clock, and your device’s audio delay is subtracted.

Further reading