func ISO8601Format(t time.Time) string {
	value := t.Format(iso8601TimeFormat)
	if len(value) < len(iso8601TimeFormat) {
		value = t.Format(iso8601TimeFormat[:len(iso8601TimeFormat)-1])
		// Pad necessary zeroes to full-fill the iso8601TimeFormat
		return value + strings.Repeat("0", (len(iso8601TimeFormat)-1)-len(value)) + "Z"
	}
	return value
}

// ISO8601Parse parses ISO8601 date string

              });
      assertEquals("Counts not as expected", Ints.asList(deltas), actualCounts);
    } finally {
      pool.shutdownNow();
    }

    // Since we have access to the backing map, verify that there are no zeroes in the map
    for (AtomicInteger value : map.values()) {
      assertTrue("map should not contain a zero", value.get() != 0);
    }
  }

  private static class MutateTask implements Callable<int[]> {