Day

A day is approximately equivalent to 24 hours.

There are approximately 1440 minutes in a day.

There are approximately 86400 seconds in a day.

There are approximately 86,400,000 milliseconds in a day.

There are 7 days in a week.

There are approximately 30 days in a month.

There are approximately 365¼ days in a year.

There are approximately 3652½ days in a decade.

There are approximately 36525 days in a century.

There are approximately 365242 days in a millennium.

Stellar and Solar Days
A Stellar day on Earth is how long it takes for the Earth to spin around once. It's approximately 23 hours, 56 minutes, and 4 seconds. However, we use Solar days instead, which are 24 hours. A solar day is longer because a solar day is how long it takes for a point on Earth to point at the sun and then spin around Earth to point at the sun again, due to the Earth orbiting around the sun a bit. These 3 minutes and 56 seconds are added because the Earth's orbit around the sun makes it so that the Earth has to spin just a little bit more to point back at the sun.

In this picture, the blue arrow on the transparent Earth is where it starts, and where it's facing.

The Blue arrow on the opaque Earth is a solar day, since it has to point back at the sun after moving, while the Pink arrow is just one rotation in a Stellar day.

Earth's rotational deceleration is (5.969 ± 1.762) × 10-7 rad yr-2.