assert(YearMonth(1999, cast(Month) 1).dayOfYear == 1);
assert(YearMonth(1999, cast(Month) 12).dayOfYear(AssumePeriod.begin) == 335);
assert(YearMonth(1999, cast(Month) 12).dayOfYear(AssumePeriod.end) == 365);
assert(YearMonth(2000, cast(Month) 12).dayOfYear(AssumePeriod.begin) == 336);
assert(YearMonth(2000, cast(Month) 12).dayOfYear(AssumePeriod.end) == 366);