The answer is not 86400

As we all know, it takes exactly 24 hours for the Sun to return to the same spot in the sky(solar rotation), which would seem obvious. 24 hours is what we think of as being a complete day, and the time it takes to transition from day to night and back again. But in truth, it actually takes the Earth 23 hours, 56 minutes, and 4.09 seconds to turn rotate once on its axis compared to the background stars(this is known as sidereal day).

This difference is probably because the Earth is orbiting around the Sun, completing one orbit in just over 365 days. If you divide 24 hours by 365 days, you’ll see that you’re left with about 4 minutes per day. In other words, the Earth rotates on its axis, but it’s also orbiting around the Sun, so the Sun’s position in the sky catches up by 4 minutes each day.

So the amount of time it takes for the Earth to rotate once on its axis is known as a sidereal day, which is exactly 23 hours, 56 minutes, and 4.09 seconds.

$(23 \times 60 \times 60) + (56 \times 60) + 4.09 =86164.09$ seconds