Convert three letter month abbreviation to number?

You can use MONTH() and create a pseudo date for it:

=MONTH(DATEVALUE(A1&" 1, 2017"))

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Or another variation on the same theme:

=--TEXT(DATEVALUE(A1 & "1, 2017"),"m")

One more note: DATEVALUE can be replaced by -- in both formula.

=MONTH(--(A1&" 1, 2017"))

and

=--TEXT(--(A1 & "1, 2017"),"m")

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