Aacini Code from here

@echo off

rem Get start time:
for /F "tokens=1-4 delims=:.," %%a in ("%time%") do (
   set /A "start=(((%%a*60)+1%%b %% 100)*60+1%%c %% 100)*100+1%%d %% 100"
)

rem Any process here...

rem Get end time:
for /F "tokens=1-4 delims=:.," %%a in ("%time%") do (
   set /A "end=(((%%a*60)+1%%b %% 100)*60+1%%c %% 100)*100+1%%d %% 100"
)

rem Get elapsed time:
set /A elapsed=end-start

rem Show elapsed time:
set /A hh=elapsed/(60*60*100), rest=elapsed%%(60*60*100), mm=rest/(60*100), rest%%=60*100, ss=rest/100, cc=rest%%100
if %mm% lss 10 set mm=0%mm%
if %ss% lss 10 set ss=0%ss%
if %cc% lss 10 set cc=0%cc%
echo %hh%:%mm%:%ss%,%cc%