code: private void recordClockSkew(long responseTimestamp) {
      if (responseTimestamp == 0L) {
        logger.debug("no date timestamp on response, cannot record skew");
      } else {
        final long delta = clock().wallTime() - responseTimestamp;
        if (delta >= 0L) {
          // Local clock is running fast compared to the server. Note this should also be the
          // common case for if the clocks are in sync as there will be some delay for the server
          // response to reach this node.
          timer(CLOCK_SKEW_TIMER, "id", "fast").record(delta, TimeUnit.MILLISECONDS);
        } else {
          // Local clock is running slow compared to the server. This means the response timestamp
          // appears to be after the current time on this node. The timer will ignore negative
          // values so we negate and record it with a different id.
          timer(CLOCK_SKEW_TIMER, "id", "slow").record(-delta, TimeUnit.MILLISECONDS);
        }
        logger.debug("clock skew between client and server: {}ms", delta);
      }
    }
docs: Record the difference between the date response time and the local time on the server.
  This is used to get a rough idea of the amount of skew in the environment. Ideally it
  should be fairly small. The date header will only have seconds so we expect to regularly
  have differences of up to 1 second. Note, that it is a rough estimate and could be
  elevated because of unrelated problems like GC or network delays.