def to_s(join: ", ", range_join: " – ", **opts)
			r=@d.map do |range|
				range.map do |d|
					DateRange.output_date(d,**opts)
				end.join(range_join)
			end.join(join)
			r.empty? ? nil : r
		end
def to_s(join: ", ", range_join: " – ", **opts)
			r=@d.map do |range|
				range.map do |d|
					DateRange.output_date(d,**opts)
				end.join(range_join)
			end.join(join)
			r.empty? ? nil : r
		end
sort_date_by :first or :last