public static String format(final Instant d) {
        return datetimeFormatter.format(d.atZone(UTC));
    }
Formats an {@code Instant} to a form of {@code 2018-03-22T00:35:44Z} at UTC.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/util/DatetimeHelper.java