Suppose, you have text denoting month in cell A1. Let's say A1 = "Sep" or A1="September", then you can use following formula to convert this to month number

=MONTH(1&A1)

=--TEXT(1&A1,"m")

In case, cell A1 contains  the partial month name say "Septe", then in place of A1 in the above formulas, you can write LEFT(A1,3).