Subtracting month from date is not giving correct value in ruby?

i am trying to subtract the month from Date its not giving accurate result

end_date = Date.parse("30-09-2019")
end_date - 1.months

returns 30 - aug - 2019

example 30-09-2019 - 1.month to give 31 - 08 - 2019
example 15-09-2019 - 1.month to give 14 - 08 - 2019