Why does my batch think that 09-01=-1? - batch-file

Question

So what I'm trying to do is create a dynamic variable that will eventually become the latest file to download from a website. In my example, I'm trying to download "downloads.informais.com/updates/psn_basic_sqlite_r4_20170531.exe" through an automated process. The r is the revision number, and the date is, well, the date. So here is what I have now:
#echo off
::http://downloads.informais.com/updates/psn_basic_sqlite_r4_20170531.exe
echo Dates > Dates.txt
::Get Variables
set num=11
set /a "num=num-1"
::: Begin set date
for /f "tokens=1-4 delims=/-. " %%i in ('date /t') do (call :set_date %%i %%j %%k %%l)
goto :end_set_date
:set_date
if "%1:~0,1%" gtr "9" shift
for /f "skip=1 tokens=2-4 delims=(-)" %%m in ('echo,^|date') do (set %%m=%1&set %%n=%2&set %%o=%3)
goto :eof
:end_set_date
::: End set date
set PSN=psn_basic_sqlite_r%num%_%yy%%mm%%dd%
set download=http://downloads.informais.com/updates/%PSN%.exe
GOTO TryPSNdownload
::Get the latest download
:TryPSNdownload
::Fix Days
if %dd%==9 set dd=09
if %dd%==8 set dd=08
if %dd%==7 set dd=07
if %dd%==6 set dd=06
if %dd%==5 set dd=05
if %dd%==4 set dd=04
if %dd%==3 set dd=03
if %dd%==2 set dd=02
if %dd%==1 set dd=01
set /a "num=num-1"
set PSN=psn_basic_sqlite_r%num%_%yy%%mm%%dd%
set download=http://downloads.informais.com/updates/%PSN%.exe
echo %download% >> Dates.txt
if exist "%download%" (
GOTO Download
) else (
::Fix Month
if %mm%==9 set mm=09
if %mm%==8 set mm=08
if %mm%==7 set mm=07
if %mm%==6 set mm=06
if %mm%==5 set mm=05
if %mm%==4 set mm=04
if %mm%==3 set mm=03
if %mm%==2 set mm=02
if %mm%==1 set mm=01
if %mm%==0 set mm=12
if %dd%==0 set dd=33
if %num%==0 set /a "dd=dd-1"
if %dd%==0 set /a "mm=mm-1"
if %num%==0 set num=11
GOTO TryPSNdownload
)
:Download
Echo %download%
pause
This is outputing the %download% variable to a txt file, so you can see what I'm talking about, but it comes out like this:
http://downloads.informais.com/updates/psn_basic_sqlite_r0_20170609.exe
http://downloads.informais.com/updates/psn_basic_sqlite_r10_201706-1.exe
http://downloads.informais.com/updates/psn_basic_sqlite_r9_201706-1.exe
I had a feeling it was an order of operations type thing, but I can't figure it out. I added the place to "fix" the dd variable, to keep it in double digits, prior to that it would count down properly, except that it wouldn't include the zero before single digit numbers. Any suggestions for how to fix?

Answer 1

I was able to fix this by changing the PSN variable to include "0" before %dd% if "dd" was less than 10, and kept the other operations the same, and now it works :-)
#echo off
::http://downloads.informais.com/updates/psn_basic_sqlite_r4_20170531.exe
echo Dates > Dates.txt
::Get Variables
set num=11
set /a "num=num-1"
::: Begin set date
for /f "tokens=1-4 delims=/-. " %%i in ('date /t') do (call :set_date %%i %%j %%k %%l)
goto :end_set_date
:set_date
if "%1:~0,1%" gtr "9" shift
for /f "skip=1 tokens=2-4 delims=(-)" %%m in ('echo,^|date') do (set %%m=%1&set %%n=%2&set %%o=%3)
goto :eof
:end_set_date
::: End set date
set PSN=psn_basic_sqlite_r%num%_%yy%%mm%%dd%
set download=http://downloads.informais.com/updates/%PSN%.exe
GOTO TryPSNdownload
::Get the latest download
:TryPSNdownload
set /a "num=num-1"
::Fix Days
if %dd% LSS 10 (set PSN=psn_basic_sqlite_r%num%_%yy%%mm%0%dd%) else (set PSN=psn_basic_sqlite_r%num%_%yy%%mm%%dd%)
set download=http://downloads.informais.com/updates/%PSN%.exe
echo %download% >> Dates.txt
if exist "%download%" (
GOTO Download
) else (
::Fix Month
if %mm%==9 set mm=09
if %mm%==8 set mm=08
if %mm%==7 set mm=07
if %mm%==6 set mm=06
if %mm%==5 set mm=05
if %mm%==4 set mm=04
if %mm%==3 set mm=03
if %mm%==2 set mm=02
if %mm%==1 set mm=01
if %mm%==0 set mm=12
if %num%==0 set /a "dd=dd-1"
if %dd%==0 set /a "mm=mm-1"
if %dd%==0 set dd=33
if %num%==0 set num=11
GOTO TryPSNdownload
)
:Download
Echo %download%
pause