SETLOCAL ENABLEDELAYEDEXPANSION
SET TODAY=%DATE%
SET YY=%TODAY:~0,4%
SET MM=%TODAY:~5,2%

IF "%MM%"=="01" (
SET /A YY=%YY%-1
SET LASTMM=!YY!12
) ELSE (
SET /A MM=%MM%-1
SET MM=0!MM!
SET LASTMM=%YY%!MM:~-2!
)
ECHO %LASTMM%