Day Converter

Exact means the value follows from the definitions, so the number shown is complete; average means the value uses a 365.25 day year.

On a clock and in this converter, a day is exactly 24 hours, or 86,400 seconds; NIST lists the day as 24 hours for use with SI units (NIST SP 811, Table 6). Real days can still differ by a second or an hour, for three reasons.

• Leap seconds. When the Earth's rotation falls behind atomic time, one second is added to UTC, so that day has 86,401 seconds. NIST lists 27 leap seconds since 1972; the last was added on December 31, 2016. In 2022 the General Conference on Weights and Measures decided that the allowed gap between UTC and the Earth's rotation will be increased in, or before, 2035 (Resolution 4).
• Daylight saving time. On the day clocks change, the local day has 23 or 25 hours, while this converter still counts every day as 24 hours.
• The Earth's actual spin. One turn relative to the stars, the sidereal day, is shorter than a clock day.

Solar Day vs Sidereal Day

A solar day runs from noon to noon and averages 24 hours. A sidereal day, one rotation measured against the stars, is about 86,164.09 seconds, or 23 hours, 56 minutes and 4 seconds (NIST SP 811, Appendix B.9). The gap of almost 4 minutes comes from the Earth's orbit: after one full rotation the Earth has moved along its path around the Sun, so it has to turn a little further before the Sun is overhead again. Astronomers use sidereal time to point telescopes, and the Sidereal Time Calculator works it out for a place and time.

Why Is a Day 24 Hours

The 24 hour day comes from ancient Egypt. Egyptian timekeepers divided daylight into 12 parts and the night into 12 more, marked by stars, and once both halves had 12 hours, the day had 24, as NIST metrologist Michael Lombardi explains in Scientific American.

A fortnight is 14 days, or two weeks. That makes it 336 hours, 20,160 minutes or 1,209,600 seconds, and about 0.459959 months on average. A pay period that comes every other week is a fortnightly, or biweekly, schedule.

Calendar months run from 28 to 31 days and years have 365 or 366 days, so any converter has to pick an average. This one uses the Julian year of 365.25 days, the average of three 365 day years and one 366 day leap year, and a month of 30.4375 days, which is that year divided by 12. Twelve of these average months make exactly one average year here, so a result never drifts when you switch between months and years.

That is why two converters can disagree in the third or fourth digit for months and years, while days, hours and weeks always agree. For a real span between two dates, where each leap day counts, use the Time Between Dates Calculator; the Leap Year Calculator shows which years have 366 days.

Single conversions that start from or end in days, one page per pair of units.

• Days to Months Converter: 1 month is 30.4375 days on average
• Days to Weeks Converter: 1 week = 7 days
• Days to Years Converter: 1 year is 365.25 days on average
• Hours to Days Converter: 1 day = 24 hours
• Days to Minutes Converter: 1 day = 1,440 minutes
• Seconds to Days Converter: 1 day = 86,400 seconds

Each converter below shows one amount of time in many units at once, starting from a different unit.

• Hour Converter: hours to minutes, seconds, days, weeks, months and years at once
• Minute Converter: minutes to hours, seconds, days, weeks and more at once
• Seconds Converter: seconds to milliseconds, nanoseconds, minutes, hours and more at once
• Milliseconds Converter: milliseconds to seconds, minutes, microseconds and more at once
• Time Unit Converter: any amount of time in 13 units, from picoseconds to decades