@Test
void testConvertWorkingDaysToDaysForHolidays() {

  Instant freitag0427 = Instant.parse("2018-04-27T19:00:00.000Z");
  Instant days = converter.addWorkingDaysToInstant(freitag0427, Duration.ofDays(0));
  assertThat(days).isEqualTo(freitag0427.plus(0, ChronoUnit.DAYS));
  days = converter.addWorkingDaysToInstant(freitag0427, Duration.ofDays(1));
  assertThat(days).isEqualTo(freitag0427.plus(3, ChronoUnit.DAYS)); // 30.4.
  days = converter.addWorkingDaysToInstant(freitag0427, Duration.ofDays(2));
  assertThat(days).isEqualTo(freitag0427.plus(5, ChronoUnit.DAYS)); // 2.5.
}