UnitConvConvert
d
Timeeverydayscientific

What is a day?

Earth's rotation period — 86,400 seconds

Definition

The day equals 86,400 seconds — the mean time for Earth to complete one rotation relative to the Sun (solar day). A sidereal day (relative to distant stars) is about 4 minutes shorter: 23 h 56 min 4 s. Most practical usage means solar day.

Common uses

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

🌍 Real-world scale

International shipping: 2–7 days typical. Global work-week: 5 days in most countries. Human circadian rhythm: 24 h ± 30 min.

Convert from days

9 conversions
Day to Second1 d = 86,400 sDay to Millisecond1 d = 86,400,000 msDay to Minute1 d = 1,440 minDay to Hour1 d = 24 hDay to Week1 d = 0.1429 wkDay to Month1 d = 0.03285 moDay to Year1 d = 0.002738 yrDay to Decade1 d = 0.0002738 decDay to Century1 d = 2.7379e-5 c

Convert to days

9 conversions
Second to Day1 s = 1.1574e-5 dMillisecond to Day1 ms = 1.1574e-8 dMinute to Day1 min = 0.0006944 dHour to Day1 h = 0.04167 dWeek to Day1 wk = 7 dMonth to Day1 mo = 30.4369 dYear to Day1 yr = 365.25 dDecade to Day1 dec = 3,652.5 dCentury to Day1 c = 36,525 d

Tips

  • 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.

Common mistakes

  • 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.

FAQ about the day

What is a day?
The day equals 86,400 seconds — the mean time for Earth to complete one rotation relative to the Sun (solar day). A sidereal day (relative to distant stars) is about 4 minutes shorter: 23 h 56 min 4 s. Most practical usage means solar day.
Where is the day used?
The day is commonly used for: Calendar dates and scheduling; Shipping and delivery times; Medical dosing intervals (e.g. "once daily").
What is an example of a day in real life?
International shipping: 2–7 days typical. Global work-week: 5 days in most countries. Human circadian rhythm: 24 h ± 30 min.
What category does the day belong to?
The day is a unit of time, part of the 10-unit time category on UnitConv.
How accurate are day conversions?
All conversion factors follow NIST and BIPM international standards to 8 significant figures — the highest practical accuracy.
Explore all 10 time units and their conversions.⏱️ Time hub →