What is a Day?

The day equals exactly 86,400 seconds (24 hours) — the mean time for Earth to complete one rotation relative to the Sun (the 'solar day'). The 'sidereal day' (relative to distant stars) is about 4 minutes shorter at 23 hours, 56 minutes, 4 seconds, but everyday usage refers to the solar day. Days are the fundamental unit of human routine: sleep cycles, work schedules, calendar appointments, and project timelines all measure in days. The day relates to the second (86,400 s = 1 day), the hour (24 h = 1 day), and the week (7 days = 1 week). Earth's rotation gradually slows due to tidal friction, lengthening the day by about 1.7 milliseconds per century — leap seconds are occasionally added to civil time to compensate, though this practice will end by 2035 by international agreement.

Visual reference for how the day relates to other time units. Each row links to the full converter for that pair.

• Calendar dates and scheduling
• Shipping and delivery times
• Medical dosing intervals (e.g. "once daily")

• ✓1 day = 24 hours = 1440 minutes = 86,400 seconds.
• ✓Julian day count is used in astronomy from 4713 BC.
• ✓A sidereal day is 23 h 56 min 4 s — slightly shorter than a solar day.

• ✗Confusing sidereal and solar days in astronomy.
• ✗Assuming a day is always exactly 24 h — leap seconds (rare) and timezone shifts vary.
• ✗Mixing business days and calendar days in contracts.