Final answer:
The SI base unit of time is the second (s), which since 1967 has been defined by the very steady vibrations of cesium atoms.
Step-by-step explanation:
The SI base unit of time is the second (abbreviated as s). Originally defined based on the mean solar day, this unit has seen a shift in definition for increased precision and reliability. In 1967, the second was redefined in terms of the vibration of cesium atoms, specifically as the duration required for 9,192,631,770 vibrations of a cesium atom to occur, which is a constant physical phenomenon unaffected by the gradual changes in Earth's rotation.