What is the base SI unit for time?

The base SI unit for time is the second. The International System of Units (SI) designates the second as the fundamental unit for measuring time due to its defined criteria based on atomic phenomena. Specifically, a second is defined by the vibration of cesium atoms, making it an extremely precise and standardized measurement.

In contrast to the second, the minute, hour, and millisecond are not base SI units; they are derived measurements. A minute consists of 60 seconds, while an hour consists of 60 minutes, and a millisecond is one-thousandth of a second. These derived units, although commonly used in everyday timekeeping, are built upon the basic measurement established by the second. Thus, the second is recognized as the foundation for expressing all other time dimensions in the metric system.