Century years are only considered as leap years if they are evenly divisible by 400. But 1600 and 2000 were leap years, because those year numbers are evenly divisible by 400. For additional information about our year, please see the exhibit on Calendars . Even though 2000 was a century year, it was also a leap year because 2000 is evenly divisible by 400. UTC is an atomic time scale, based on the performance of atomic clocks.
Astronomical time is based on the rate of rotation of the earth.
Since atomic clocks are more stable than the rate at which the earth rotates, leap seconds are needed to keep the two time scales in agreement. Based on what we know about the earth's rotation, it is unlikely that we will have a negative leap second in the foreseeable future. Currently the difference between UT1 and UTC is changing at a rate of about 2 to 3 milliseconds per day, which makes a leap second necessary at an average interval of slightly more than 1 year. Historically, leap seconds have only been implemented on June 30th or December 31st. Here is the current difference between UT1 and UTC and a list of all leap seconds .