The change in summer 2025 time approaching in France, with its share of questions and comments. However, the so -called “legal” time is unknown …
With the succession of time changes each year and the end of the emblematic talking clock less than two years ago, in the summer of 2022, we almost forgot it. Legal time in mainland France is defined by a super clock located at the Paris Observatory. Several ultra precise atomic clocks in reality, capable of establishing the exact time to the billionth of a second. If everyone will wonder this weekend the hour with the time 2025 time, which comes in the night from Saturday to Sunday, little will know that it is this device that changes the first to go to the summer time.
Forgotten in the daily life, the legal time is however essential in ministries, town halls, schools, swimming pools, but also industry, transport, telecoms, or stock market transactions. It allows all actors to coordinate and, in a way, avoids disorder or even disasters. At the Paris Observatory, these increasingly precise clocks transmit the time everywhere in France via a giant antenna of 350 m. A large wave transmitter, located in Allouis, in the Cher, and controlled by the Observatories of Paris and Besançon, which broadcasts the “ALS 162 time signal”, throughout France, buildings and tunnels included.
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How to know the legal time in France?
Next to 200,000 receivers, dedicated clocks installed in public places across the territory, which are automatically settled at legal time, via radio signal. Municipalities have chosen to connect their bell towers, Beffrois, town hall clocks … to no longer have to perform human intervention during the time change and always display a 100%guaranteed hour.
It is normally on this hour that your clocks and watches must be modeled. Since the end of the speaking clock, the legal time is also available on the internet. Developed in the spring of 2022 in anticipation of the judgment of the good old service by telephone, this online service allows everyone to know the time issued from the Paris Observatory, to the nearest second.
The principle is simple: by connecting to the site from a computer, a smartphone or a tablet, the user can immediately view the legal time in force in mainland France. The site also indicates whether we are currently in winter or in summer time, a parameter that influences the lag with the coordinated universal time (UTC).
To provide an hour of optimal clarification, the “French Legal Hour” site implements a clever synchronization process. The Paris Observatory server recovers the time from the time from the device of the Internet user that connects. He then compares this statement with his own time reference, while taking into account the round-trip transmission time of the request on the network. A correction is then made to erase the desynchronization between the two systems.
After the date of the time change, pay everything at summer time … or not
Transparent for the user, regular exchanges between their terminal and the servers of the observatory are made over the navigation on the site. Each time, the time difference is measured and compensated, to permanently display an updated legal hour, reliable to the nearest thousandth of a second. On the date of time change, the tool will therefore be most useful for adjusting all your devices.
This synchronization mechanism is inspired by the well -known protocol of computer networks: the NTP (Network Time Protocol). Moreover, the Paris Observatory also provides publicly accessible time servers, which allow network administrators to align the internal clocks of their materials in the French legal time. And to your smartphone, your watch or any other connected object, to save this setting after the time change.