2.3. Is UTC the same as GMT?
The observatory in Greenwich derived GMT from astronomical events like the solar day. UTC is based on a quantum resonance of a cesium atom, being quite more accurate.
Unfortunately the earth's rotation is not very much impressed by the definition of the UTC second. Having 86400 UTC seconds per day on an earth that's slowing down would mean that midnight would eventually fall in the middle of the day. As this is probably unacceptable, some extra seconds can be added or removed inside the UTC time-scale to keep synchronization. That patch work is named leap seconds.
To make things worse, leap seconds can be predicted as much as the earth's rotation, which is not at all. Therefore you can't easily make calculations for dates in the future using UTC; at least not with accuracy of a few seconds.
2.4. What happens during a Leap Second?
During a leap second, either one second is removed from the current day, or a second is added. In both cases this happens at the end of the UTC day. If a leap second is inserted, the time in UTC is specified as 23:59:60. In other words, it takes two seconds from 23:59:59 to 0:00:00 instead of one. If a leap second is deleted, time will jump from 23:59:58 to 0:00:00 in one second instead of two.
I've had some posts here about certain fun dates that have come and gone. It finally got to the point where so many were coming by in the early part of this millennium that I lost interest in the subject.
I ran across the information above which I found amusing. Apparently Universal Coordinated Time isn't particularly constant. I'm not sure how your average computer knows that a leap second has occurred without contacting a smart time server - probably the feeling is that it doesn't matter too much, even though UTC is used by Windows to store the time when a file was created or modified.