The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France.
This was the reason why in 1889 by the 1st General Conference on Weights and Measures (CGPM) of the Bureau International des Poids et Mesures (BIPM) the three characteristics length, mass, and time were selected and in 1954 during the 10th CGPM three more characteristics were added and it was decided to derive an International System of Units (SI) that should cover all physical characteristics.
The measurement of time is overseen by BIPM (Bureau International des Poids et Mesures), located in Sèvres, France, which ensures uniformity of measurements and their traceability to the International System of Units (SI) worldwide.