Batch bat calculates the date of the first day of N months before N months

 @echo off
rem calls GNU for Win32
set "gnuDate=C:/test/"
rem Specify the number of months
set MonthsAgo=1
for /f %%a in ('%gnuDate% -d "%MonthsAgo% months ago" "+%%Y-%%m-01"') do (
    set "DstDate=%%a"
    )
)
echo,%DstDate%
pause
 

@echo off
rem Specify the number of months
set MonthsAgo=2
>"%temp%/" echo strLastDate=DateAdd("m", -%MonthsAgo%, Date)
>>"%temp%/" echo strFmtDate=Right(Year(strLastDate),4) ^& Right("0" ^& Month(strLastDate),2) ^& "01"
>>"%temp%/" echo strFmtDate
for /f %%a in ('cscript /nologo "%temp%/"') do (
    set "DstDate=%%a"
)
set DstDate=%DstDate:~0,4%-%DstDate:~4,2%-%DstDate:~6,2%
echo,%DstDate%
pause