According to this calendar, every year divisible by four has an extra day. However, "centurial years" (those ending in double zeros) would have an extra day only if they were divisible by 400. Thus, the year 2000 was a leap year, bu the year 1900 was not.
In order to keep the atomic clock in sync with the earth's rotation, occasionally a "leap second" is added to the day.
In 2015, a leap second was added on June 30.
- Little Black Book, Diocese of Saginaw