De acordo com este site , devo alterar minha fórmula para =IF(A1="","-",DATEDIF($A1,$B1+15,"m"))
Rounding results up or down
The DATEDIF() function always rounds down (by default) to the nearest >whole month or year.
If you want to calculate months or years rounded up, add half a month or >half a year to the formula like this: =DATEDIF(A19,B19+15,”m”) for half a >month (or 15 days) or =DATEDIF(A21,B21+183,”y”) for half a year (or 183 >days). The result is now rounded up to the nearest month or year.