How can I calibrate or verify a chronometer?

If you measure time with a required uncertainty of a few tenths of a second, then a calibration with the speaking clock is perfectly suitable.

InFrance, dial 3699 for the speaking clock. The speaking clock is controlled by LNE-SYRTE, which is tasked by the French national metrology and test laboratory (LNE) with managing the national time and frequency references. Therefore, it can be traced to the national measurement standards.