Excel’s `DATEDIF`

function (with the option to return the number of months) returns different values for two pairs of date values even though the dates are in the same months.

In column C, I use `=IF(A1="","-",DATEDIF($A1,$B1,"m")) + 1`

```
A B C
10-Nov-17 9-May-21 42
10-Nov-17 10-May-21 43
```

My question:

How to make the C2 return `42`

?

How to make column C return `42`

without using `+1`

in the end of the formula?