def +(other)
      other = other.to_days if other.respond_to?(:to_days)
      ISO8601::Date.new((@date + other).iso8601)
    end
def +(other)
      other = other.to_days if other.respond_to?(:to_days)
      ISO8601::Date.new((@date + other).iso8601)
    end
Forwards the date the given amount of days.

 @param [Numeric] other The days to add

 @return [ISO8601::Date] New date resulting of the addition