I really liked Joey's method, but I thought I'd expand upon it a bit.
In this approach, you can run the code multiple times and not worry about the old date value "sticking around" because it's already defined.
Each time you run this batch file, it will output an ISO 8601 compatible combined date and time representation.
FOR /F "skip=1" %%D IN ('WMIC OS GET LocalDateTime') DO (SET LIDATE=%%D & GOTO :GOT_LIDATE)
:GOT_LIDATE
SET DATETIME=%LIDATE:~0,4%-%LIDATE:~4,2%-%LIDATE:~6,2%T%LIDATE:~8,2%:%LIDATE:~10,2%:%LIDATE:~12,2%
ECHO %DATETIME%
In this version, you'll have to be careful not to copy/paste the same code to multiple places in the file because that would cause duplicate labels. You could either have a separate label for each copy, or just put this code into its own batch file and call it from your source file wherever necessary.