A day contains 24 hours which are being represented by 60 minutes each. One minute consists 60 seconds.
On the other hand, an hour is just a port of a bigger unity. 7 days are a week and approximately 4 weeks form a month, exactly 12 months are a year. A year consists of 365 days and is meant to describe the amount of time the earth needs to go fully round the sun once. But the Earth needs a little bit more time for this. To compensate this inacurancy, every 4th year is being counted with 366 days. This additional day is added to the second month of the year. Exceptions to this rule are those years which change »hundred« digit. According to the mathematics, they should contain 366 days, but the contain just the normal amount of 365 days.
You see, the day is some kind of border between acuracy and inacuracy.