Scripting adding 8 minutes to a timestamp

Hi
I have the following script generating a time stamp which is used to name a file.
I like to add 7 extra minutes to any generated time stamp.

Here is the code:
:getTimeStamp
Set stmp=
For /f "tokens=* skip=1" %%a in (' wmic os get localdatetime ') do (
Set stmp=%%a
Set stmp=!stmp:~0,8!_stmp:~8,4!
)
Then I set my folder name using this time stamp:
Set fildername= xxxxx_ %smtp% however I like to add 5 minutes to this %smtp% later. How can it be done?

Thanks in advance