Puzzle ZBOM 

Workings

Hint
Look at the first letters of the months.

Answer
199.

Take the first letter of the month, and find its position in the alphabet: S = 19.

Then take the number of letters in the month: September = 9.
January    10-7
February   6-8
March      13-5
April      1-5
May        13-3
June       10-4
July       10-4
August     1-6
September  19-9