Answer: Caesium

The exact definition of a second has undergone quite a few revisions over the centuries. The second was originally defined as the second division of the hour by 60 (the first division was the minute).

Loose time keeping well into the 19th century however meant that a second was fairly fluid if it was even measured at all. By 1892 scientists had finally hammered out a more precise definition: the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

That definition was refined for next seventy years or so until the  International Committee for Weights and Measures abandoned the idea of linking the scientific second to explicitly to length of the day. Instead the second would be defined via atomic clock, specifically: the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

Although atomic clocks have been improved many fold since the 1960s, isotopes of the caesium atom have been keeping time for us ever since.