=INT(a1/12) &";"& MOD(a1, 12)
Where a1 is the cell that contains your months.
INT takes a number and rounds it down. So if A1 was 15 then 15/12=1.25 rounded down would be 1.
MOD returns the remainder after a number is divided by a divisor (in this case, 12).
This is long division which I am sure most people have forgotten once you reach a certain age! Although understanding how it works isn’t completely neccessary it doesn’t hurt to. See this link for an explanation.