Programmer - windows batch script format date and time

Programmer Question

In a windows (XP) batch script I need to format the current date and time for later use in files names etc.,

Similar to http://stackoverflow.com/questions/864718/how-to-append-a-date-in-batch-files but with time in as well

I have this so far:

echo %DATE%
echo %TIME%
set datetimef=%date:~-4%_%date:~3,2%_%date:~0,2%__%time:~0,2%_%time:~3,2%_%time:~6,2%
echo %datetimef%

which gives:

28/07/2009
 8:35:31.01
2009_07_28__ 8_36_01

Anyway a can allow for non-24 hour in %TIME% so I can get:

2009_07_28__08_36_01