auto yq0 = YearMonth(2020, Month.jan);

yq0.add!"quarters"(1);
assert(yq0.year == 2020);
assert(yq0.month == Month.apr);

auto yq1 = yq0.add!"quarters"(1);
assert(yq1.year == 2020);
assert(yq1.month == Month.jul);

// also changes yq0
assert(yq0.year == 2020);
assert(yq0.month == Month.jul);

yq1.add!"quarters"(2);
assert(yq1.year == 2021);
assert(yq1.month == Month.jan);

yq1.add!"quarters"(-5);
assert(yq1.year == 2019);
assert(yq1.month == Month.oct);