The fifth hyperfactorial: 5⁵×4⁴×3³×2²×1¹=86400000 milliseconds is exactly 24 hours. 24 hours is approximately 1 earth day.
Or does that count? Is that caused by a partial day in the revolution of the earth around the sun or a failure of a day to fit into exactly 24 hours? Or both? Are we talking solar or sidereal days?
Leap years have nothing to do with milliseconds in a day- it’s days that fail to fit into a year; days and years are defined by the orbit and rotation of the earth. Leap seconds, however, are another story...