Estimate duration of "dir c:\windows\system32 /s" command

set t1=%time%
echo Started at %t1%
dir "%windir%\system32" /s
set t2=%time%
set /A dt=(%t2:~6,2%-%t1:~6,2%)+(60*(%t2:~3,2%-%t1:~3,2%))+(3600*(%t2:~0,2%-%t1:~0,2%)) > nul
echo Started at %t1%, finished at %t2%
echo Duration %dt% seconds