Final answer:
The standard clock is zeroed with universal time (UT) or Greenwich Mean Time (GMT). Physicists use cesium clocks to achieve accuracy in modern clocks.
Step-by-step explanation:
The standard clock is zeroed with universal time (UT), also known as Greenwich Mean Time (GMT). Universal time is the local time in Greenwich, England, at the zero meridian. It serves as a single agreed-on clock that marks time worldwide and is not tied to any specific locality.
Physicists use cesium clocks to achieve the accuracy required for modern purposes. The cesium atom is used as a standard to calibrate clocks. Supercooled cesium atoms are bombarded with carefully controlled microwaves, and when the microwave frequency is exactly right, the atoms absorb energy and undergo an electronic transition. The frequency serves as the clock's pendulum.
It's worth noting that there are also examples, like a stopped or broken clock, where the time it shows may happen to be correct even though the clock itself is not functioning properly. However, these are exceptions to the standard clock mechanism.