A day is conventionally defined as 24 hours long. However, the exact length of a day varies slightly due to Earth's rotation and orbital dynamics. A solar day, which is the time it takes for the Sun to return to the same position in the sky, is on average 24 hours or 86,400 seconds. But in reality, a day is not always precisely 24 hours; it varies by a few milliseconds or seconds throughout the year. There is also the concept of a sidereal day, the time it takes Earth to rotate 360 degrees relative to distant stars, which is about 23 hours, 56 minutes, and 4 seconds. This differs from the solar day because Earth moves a bit in its orbit around the Sun while rotating. Modern timekeeping rounds the day to 24 hours, but small variations occur due to Earth's changing speed and tidal forces, sometimes even requiring "leap seconds" to keep clocks accurate. The length of a day also gradually increases over long periods due to the Moon's tidal friction slowing Earth's rotation. In summary:

• A standard day is 24 hours (86,400 seconds) by definition.
• Actual solar day length varies slightly by milliseconds throughout the year.
• A sidereal day is about 23 hours, 56 minutes, and 4 seconds.
• Variations are accounted for with leap seconds and precise timekeeping systems.