@VisibleForTesting
    String currentTimestamp() {
        final long currentTimeNanos = nanoTimeSupplier.getAsLong();

        if (nextUpdateNanos - currentTimeNanos > 0) {
            return timestamp;
        }

        final Instant now = instantSupplier.get();

        // The next time we need to update our formatted timestamp is the next time System.nanoTime()
        // equals the following second. We can determine this by adding one second to our current
        // nanoTime minus the nanos part of the current system time (i.e., the number of nanos since the
        // last second).
        nextUpdateNanos = currentTimeNanos - now.getNano() + NANOS_IN_SECOND;

        return timestamp = DateTimeFormatter.RFC_1123_DATE_TIME.format(
                ZonedDateTime.ofInstant(now, ZoneOffset.UTC));
    }
Returns the HTTP formatted timestamp for the current time.
core/src/main/java/com/linecorp/armeria/internal/common/util/HttpTimestampSupplier.java