SetUTCMonth returns weird result

Question

When the current date is 31st of any month , setUTC date is jumping to next to next month .
Example :
Var cdate = new Date(‘August 31, 2018’) ;
cdate.setUTCMonth(8);
Now try to getUTCMonth(9) and check .. it is returning October .

Answer 1

September 2018 have not a 31th day then js make your date like you want increment by one day (going to first october)

Answer 2

when i use setUTCyear(x,y,z) then it is returning fine ..