Returns a monotonic clock measurement, if supported by the underlying platform.

This is the pure equivalent of Java's System.nanoTime, or of CLOCK_MONOTONIC from Linux's clock_gettime().

clock.monotonic(NANOSECONDS)

The returned value can have nanoseconds resolution and represents the number of time units elapsed since some fixed but arbitrary origin time. Usually this is the Unix epoch, but that's not a guarantee, as due to the limits of Long this will overflow in the future (2⁶³ is about 292 years in nanoseconds) and the implementation reserves the right to change the origin.

The return value should not be considered related to wall-clock time, the primary use-case being to take time measurements and compute differences between such values, for example in order to measure the time it took to execute a task.

As a matter of implementation detail, the default Clock[IO] implementation uses System.nanoTime and the JVM will use CLOCK_MONOTONIC when available, instead of CLOCK_REALTIME (see clock_gettime() on Linux) and it is up to the underlying platform to implement it correctly.

And be warned, there are platforms that don't have a correct implementation of CLOCK_MONOTONIC. For example at the moment of writing there is no standard way for such a clock on top of JavaScript and the situation isn't so clear cut for the JVM either, see:

• bug report
• concurrency-interest discussion on the X86 tsc register

The JVM tries to do the right thing and at worst the resolution and behavior will be that of System.currentTimeMillis.

The recommendation is to use this monotonic clock when doing measurements of execution time, or if you value monotonically increasing values more than a correspondence to wall-time, or otherwise prefer realTime.

Returns the current time, as a Unix timestamp (number of time units since the Unix epoch), suspended in F[_].

This is the pure equivalent of Java's System.currentTimeMillis, or of CLOCK_REALTIME from Linux's clock_gettime().

The provided TimeUnit determines the time unit of the output, its precision, but not necessarily its resolution, which is implementation dependent. For example this will return the number of milliseconds since the epoch:

import scala.concurrent.duration.MILLISECONDS

clock.realTime(MILLISECONDS)

N.B. the resolution is limited by the underlying implementation and by the underlying CPU and OS. If the implementation uses System.currentTimeMillis, then it can't have a better resolution than 1 millisecond, plus depending on underlying runtime (e.g. Node.js) it might return multiples of 10 milliseconds or more.

See monotonic, for fetching a monotonic value that may be better suited for doing time measurements.