The ultra-short variant of what is going on: Time in Erlang was historically used for many things: What time is it? How much time elapsed between points A and

The ultra-short variant of what is going on: Time in Erlang was historically used for many things: What time is it? How much time elapsed between points A and B? Give me time, use it as a unique timestamp. And so on. The old way Erlang handled time (with time correction enabled) was to speed up or slow down the internal clock by about 1% whenever the system time changed. This is fine for smaller fluctuations like leap seconds (and correspond nicely to what Google calls leap-second-smearing). But for a large sudden "time warp", the system will be wrong forever because it will never catch up. The 1% frequency difference also means that 1000ms is closer to 990ms or 1010ms. The new API maintains a precise and accurate monotonic clock which is used to wait on timers, do latency calculations and such. And then a "system time" is kept by noting how the system time is "offset" from the internal monotonic clock. You can monitor time change events of this "system" time, which allows your program to explicitly handle leap seconds, or when the system suddenly gets full NTP connectivity and learns that 14 days has passed since last reboot. Finally, a seperate API can give unique integers. Earlier, erlang:now() was guaranteed unique (even if called from several different processes at the same time. It would simply "speed up" for a while if the calls were too close to each other). The downside of this guarantee was a big fat lock around erlang:now(), which has been dreadful to anyone who wanted good scalability in their programs. Lock instrumentation could often show serious contention around this lock. By querying the underlying OS for the best clock source and separating the different time concepts, timer wheels can now be per-core, which results in a massive scalability boost of the system as a whole. All in all, it is a necessary and very cool change to Erlang systems. It allows one to handle time with the attention to detail which is needed. While the original leap-smearing tactic is fine (and worked well for a long time), this finally allows people to program real solutions to leap seconds and sudden NTP time warps. (Edit: added a "never" which was needed for a sentence to parse correctly).