A day is commonly defined as 24 hours for civil timekeeping, corresponding to one complete rotation of the Earth relative to the Sun (a solar day). However, there are related concepts that refine this number:

• Sidereal day: about 23 hours 56 minutes, measuring the Earth's rotation relative to distant stars.

• Leap seconds: occasionally added to UTC days to keep atomic time in sync with Earth's slowing rotation, resulting in occasional 23-hour or 25-hour civil days in a given locale or year.

• Variations over time: Earth's rotation gradually slows due to tidal interactions, so the length of a solar day can vary by a few milliseconds from day to day, though 24 hours remains the standard civil measure.

Short answer: for everyday use, a day is 24 hours long. If you’re thinking in astronomical terms, a sidereal day is approximately 23 hours 56 minutes, and if you’re aligning clocks to atomic time, you use exact 86,400 seconds per SI day with occasional leap seconds to keep pace with Earth's rotation.