Year B.C. of the Gregorian Calendar counting year 0 as 1 B.C.
std.datetime.date.DateTimeException if isAD is true.
assert(Date(0, 1, 1).yearBC == 1); assert(Date(-1, 1, 1).yearBC == 2); assert(Date(-100, 1, 1).yearBC == 101);
See Implementation
Year B.C. of the Gregorian Calendar counting year 0 as 1 B.C.