Time doesn't exist. It is a concept created by the human mind in order to make sense of what we do in terms of placing it into a sequence of events. It also helps in terms of commercial activities. The fact that we have 24hours in a day, 60minutes in a hour, and 60seconds in a minute is effectively a made up system. Sure we measure a second by so many vibrations of a caesium atom, but that's just so we have a constant. It's like the concept of a metre. You can actually go and see the original bar which was decided would be used as the basis of the metric system. The length of this bar was called a metre, which was then divided into 100cm and 1000mm.
As to why the book says 23 hours 56minutes, that's because we have a time system which is not reliant on the earth's rotation around the sun, which varies ever so slightly due to reasons stated by Tom Allen. This figure is an average.
As to why the book says 23 hours 56minutes, that's because we have a time system which is not reliant on the earth's rotation around the sun, which varies ever so slightly due to reasons stated by Tom Allen. This figure is an average.