Periodically the International Earth Rotation Service determines that Coordinated Universal Time needs to be adjusted to match the mean solar day. This is because the earth doesn’t rotate at a constant speed and in general takes slightly longer than 86,400 seconds. These adjustments have been done through adding (or, in theory, removing) a second every few years. The next leap second insertion is scheduled for June 30th, 2015 at 23:59:60 UTC. How does GemStone/S handle that?
GemStone gets time from the host OS (Unix Time) and is typically described as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970. This definition is correct so long as we assume that each and every day has exactly 86,400 seconds. If you take into account the 27 leap seconds that will have been added as of June 30, 2015, it is more accurate to say that Unix Time is the number of atomic seconds that have elapsed since 00:00:27 UTC, 1 January 1970. The way the adjustment occurs is that when there is a leap second, the Unix time counter takes two atomic seconds to advance by one Unix second.
The impact of this approach is that a record of when something happened will be correct, but the time between two events could be reported as being one or more seconds less than the actual time between the events. For example, Unix time (and GemStone) will report that there are five seconds between June 30 at 23:59:55 and July 1 at: 00:00:00 when in fact in 2015 there were six seconds between those two times.
Whether this matters is an application-level or domain-level problem. If keeping track of the time between events needs to be accurate (e.g., recording some rapid physics event), then relying on Unix time (or GemStone) will not be sufficient. If all that is needed is a timestamp and it is acceptable to have June 30, 2015 23:59:59 UTC last for 2000 milliseconds, then things should be fine.