# Difference in days between `self` and `other`
	fun diff_days(other: Date): Int
	do
		var y_out = year - other.year
		y_out = y_out * 365
		var m_out = month - other.month
		m_out = m_out * 30 # FIXME
		return day - other.day + m_out + y_out
	end