Day (d)

Definition

Exactly 24 hours (86 400 seconds).

History

The day is based on one rotation of the Earth and is fixed for timekeeping at exactly 24 hours, or 86 400 seconds. The natural day varies slightly, so a mean value is used.

Where it is used

Days are the basic unit of the calendar and of everyday durations.

Typical uses include calendars, durations.

Conversion pages

These pages convert day to or from another unit in time.

No conversion pages use this unit yet.

Other Time units

• Second (s)
• Minute (min)
• Hour (h)
• Week (wk)
• Month (mo)
• Year (yr)

Sources

• BIPM SI BIPM SI Brochure (2019); the second is defined by the caesium-133 transition · Dataset reviewed Sep 18, 2026