The time on your watch feels absolute, but the official time the world runs on is a masterpiece of scientific compromise, constantly being adjusted to account for the fact that the Earth itself is an imperfect clock.

Is Time Slipping? The Truth About the Earth's Rotation

For centuries, time was based on the Earth's rotation relative to the sun. However, the Earth's spin isn't perfectly uniform. It is gradually slowing down due to tidal friction from the moon, and it can also be affected by events like major earthquakes or movements in the Earth's core. This creates a conflict between the time our planet's rotation dictates and the time kept by hyper-precise atomic clocks.

Atomic Time (TAI) vs. Universal Time (UT1): The Core Conflict

Modern timekeeping is a negotiation between two standards:

• International Atomic Time (TAI): This is the output of hundreds of atomic clocks around the world. It is incredibly precise and stable, losing or gaining less than a second over millions of years. It is pure, uninterrupted time.
• Universal Time (UT1): This is an astronomical time standard based on the actual rotation of the Earth. It is necessary for navigation and astronomy, as it corresponds to the position of the sun and stars in the sky.

The Politics of the Leap Second

Because the Earth's rotation (UT1) is slowing down relative to atomic time (TAI), the two standards drift apart. To keep them aligned, the world's timekeeping authorities have periodically added a leap second to Coordinated Universal Time (UTC), the global civil time standard. This addition, usually on June 30th or December 31st, creates a minute with 61 seconds. However, this practice is highly controversial. It can cause software bugs in systems that aren't prepared for it, and there is a major international debate on whether to abolish the leap second entirely and let the two time standards slowly drift apart.

GMT vs. UTC: What's the Difference (and Does It Matter)?

For most people, Greenwich Mean Time (GMT) and Coordinated Universal Time (UTC) are interchangeable. GMT is an older standard based on the mean solar time at the Royal Observatory in Greenwich, London. UTC is the modern, more precise standard based on atomic time, but it is kept within 0.9 seconds of UT1 by the addition of leap seconds. In casual use, the difference is negligible, but in scientific and technical fields, UTC is the correct and only standard.

Measure the Tiniest Differences in Time

While the world debates leap seconds, you can measure your own moments with millisecond precision.

Further Reading

• For a complete overview, see The Complete Time Calculator Resource.
• Explore all date tools in The Ultimate Date Calculator Hub.