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Minutes, hours, and days are the three most basic time units used in everyday life and scientific measurement. A minute is defined as 60 seconds, an hour consists of 60 minutes, and a day is traditionally considered as 24 hours. These units are derived from the International System of Units (SI) where the second is the base unit of time. The conversion factors between them are fixed: one hour equals 60 minutes or 3,600 seconds; one day equals 24 hours, 1,440 minutes, or 86,400 seconds.

In civil timekeeping, the day is divided into two 12‑hour periods labeled AM and PM to reflect the rotation of the Earth on its axis. The transition between days, known as midnight, is marked at 00:00 hours in the 24‑hour clock system commonly used in scientific, military, and aviation contexts. Weeks, months, and years are long‑term units that are built from days, although their lengths vary due to the lunar and solar cycles.

Chronology of historically significant time systems shows how different civilizations adopted distinct minute, hour, and day structures. For example, the ancient Roman hour was unequal, changing with daylight, while the medieval Muslim calendar introduced 24 equal hours, leading to the standardization of the modern 24‑hour day. Contemporary digital technology relies on these standardized units for timestamping, scheduling, and coordinating global events.