다섯달/오달?
달 means month isn't it?
일, 월, 년 using sino korean
then does 달 using sino korean too?

Question

다섯달/오달?
달 means month isn't it?
일, 월, 년 using sino korean
then does  달  using sino korean too?

Answer

5월(오월) -> May

다섯 달, 오개월-> five months (it means a period)

Answer

If you want to say with the word "다섯", it will be 다섯달.
If you want to say with the word "오", it will be 오개월.

Both 달 and 개월 refer to a month.

년 (year) is only used with 오 (일, 이, 삼, 사...), not 다섯.(하나, 둘, 셋, 넷..)

I hope it helps :)