A day is 24 hours long on average, equal to 86,400 seconds. However, there are nuances:

• Apparent solar day: the time from one solar noon to the next, which averages about 24 hours but can vary by a few seconds due to Earth's irregular rotation and orbital dynamics.
• SI day: defined exactly as 86,400 seconds for timekeeping and computer standards.
• Sidereal day: about 23 hours 56 minutes 4 seconds, measured relative to distant stars, not the Sun.
• Civil days can be 23, 24, or 25 hours in some places when daylight saving time is observed, though the standard day length remains 24 hours in terms of seconds.

If you’re after a precise, context-specific length (solar, sidereal, or civil day) for a given date or location, tell me which type you need and I’ll provide the exact figure for that scenario.