00:00 hours

In 24-hour time, 12:00 a.m. midnight is not 24:00 hours, it is 00:00 hours.

23:00 hours

In 24-hour time, 11:00 p.m. is 23:00 hours.

The 21st century

We are currently living in the 21st century.

Seconds

Sixty seconds are in each minute of time.

Millennium

A millennium is equal to one thousand years.

Century and decade

A century is equal to one hundred years. A decade is equal to ten years.