BrainBashers - Puzzle ZJSA

The legendary BrainBashers calendar has had a small problem.

Here is a listing showing the number of days in each month:
January    73
February   83
March      51
April      52
May        31
June       42
July       41
August     63
September  ?

Using the same rules, how many days are in September?

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Hint
The first digit represents something, the second digit represents something else.