Gets the current millisecond instant of the clock.
This returns the millisecond-based instant, measured from 1970-01-01T00:00Z (UTC). This is equivalent to the definition of {@link System#currentTimeMillis()}.
Most applications should avoid this method and use {@link Instant} to representan instant on the time-line rather than a raw millisecond value. This method is provided to allow the use of the clock in high performance use cases where the creation of an object would be unacceptable.
The default implementation currently calls {@link #instant}.
@return the current millisecond instant from this clock, measured fromthe Java epoch of 1970-01-01T00:00Z (UTC), not null
@throws DateTimeException if the instant cannot be obtained, not thrown by most implementations