@Test
void YearMonth로_직전월을_구한다() throws Exception {

    //given
    YearMonthRequestDto dto = new YearMonthRequestDto(of(2020,9));
    //whens
    YearMonth beforeMonth = dto.getBeforeMonthByYearMonth();
    //then
    assertThat(beforeMonth).isEqualTo(of(2020,8));
}