code: public static ZonedDateTime now(Clock clock) {
          Jdk8Methods.requireNonNull(clock, "clock");
          final Instant now = clock.instant();  // called once
          return ofInstant(now, clock.getZone());
      }
docs: Obtains the current date-time from the specified clock.
  <p>
  This will query the specified clock to obtain the current date-time.
  The zone and offset will be set based on the time-zone in the clock.
  <p>
  Using this method allows the use of an alternate clock for testing.
  The alternate clock may be introduced using {@link Clock dependency injection}.
  
  @param clock  the clock to use, not null
  @return the current date-time, not null