Our clocks adjust to match the 24-hour day through the use of coordinated timekeeping systems, primarily based on the concept of leap seconds. Leap seconds are added periodically to ensure that our atomic clocks, which are extremely accurate, stay synchronized with the Earth's rotation, which is not perfectly consistent.
The Earth's rotation, due to various factors like tidal forces and the redistribution of mass, slows down or speeds up slightly over time. This means that the length of a day can vary by milliseconds. While this discrepancy may seem small, over an extended period, it can accumulate and cause a significant difference between atomic time and the Earth's rotational time.
To account for this difference, the International Earth Rotation and Reference Systems Service (IERS) closely monitors the Earth's rotation and determines when a leap second is necessary. Leap seconds are added to Coordinated Universal Time (UTC), which is the international standard for civil timekeeping. UTC is used as the basis for setting local times around the world.
When a leap second is required, it is inserted at the end of either June 30th or December 31st. For example, if the Earth's rotation has slowed down, an additional second is added at the end of June 30th, adjusting the time to 23:59:60 before rolling over to the next day. This allows the atomic clocks to catch up to the Earth's rotational time.
By periodically adding or subtracting leap seconds, our atomic clocks, which are highly precise and consistent, can be kept in sync with the Earth's rotation. This adjustment ensures that our clocks, including those used for everyday purposes, scientific measurements, and global timekeeping systems, continue to align with the concept of a 24-hour day as determined by the Earth's rotation.