leap second : Java Glossary

Because leap seconds can’t be predicted, Java ignores them entirely. You deal with them by readjusting your computer clocks from precise atomic clocks on the web that know about the leap seconds, then pretending they never happened. For most civil purposes, this suffices. You could not get away with that sloppiness if you were implementing GPS (Global Positioning System), plotting satellite orbits, or doing astronomy.

GPS uses a SI (Systèm Internationale (metric)) clock without the leap seconds.

Leap seconds are bailing wire. They foul technology. The problem is, every once is a while you get two seconds with the same label, much the way in fall you get two hours from 1AM to 2AM confounding schedules and labour records. Communication satellites are confused and thrown off base for a few seconds. The International Bureau of Standards is considering abolishing the leap second, replacing it with continuous clock adjustment. I gather satellites would have two clocks, one unadjusted atomic and one continuously synchronised to the earth’s rotation.