Historically, time was based on the mean rotation of the Earth relative to celestial bodies and the second was defined from this frame of reference. But the invention of atomic clocks brought about a definition of a second that is independent of the Earth's rotation and based on a regular signal emitted by electrons changing energy state within an atom.
In 1970, an international agreement established two timescales: one based on the rotation of the Earth and one based on atomic time.
The problem is that the Earth is very gradually slowing down, continually throwing the two timescales out of synch, so every so often, a "leap second" has to be tacked on to the atomic clock.
Someone else: You're late!
This guy: How many active hydrogen masers have you got at home, motherfucker?
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