Returns the day-of-month (1-31) as a `ubyte` from a `Date`.
auto d = Date(2020, Month.may, 31); assert(d.year == 2020); assert(d.quarter == Quarter.q2); assert(d.month == Month.may); assert(d.day == 31);
Returns the day-of-month (1-31) as a `ubyte` from a `Date`.