Day (d)
The day, symbol d, is the time it takes Earth to rotate once. A day is 24 hours or 86,400 seconds. Because Earth’s spin changes a little, days can vary, so leap seconds are sometimes added to atomic clocks. The day is a key part of human life, shaping calendars, routines, and events in all cultures. It is one of the most fundamental and widely recognized time units.
Microsecond (µs)
The microsecond, symbolized as µs, is one millionth of a second. This very short time is important in electronics, communication, and research. Microseconds are used to measure signal delays, computer speeds, and fast changes. As digital technology grew in the 1900s, measuring microseconds became necessary. Today, this accuracy supports applications such as fast trading, medical imaging, and satellite communications.