auto ym0 = YearMonth(2020, Month.jan); ym0.add!"years"(1); assert(ym0.year == 2021); assert(ym0.month == Month.jan); auto ym1 = ym0.add!"years"(1); assert(ym1.year == 2022); assert(ym1.month == Month.jan); // also changes ym0 assert(ym0.year == 2022); assert(ym0.month == Month.jan);