As others have already pointed out, the date and time formats of %DATE% and %TIME% (as well as date /T and time /T) are locale-dependent, so extracting the current date and time is always a nightmare, and it is impossible to get a solution that works with all possible formats since there are hardly any format limitations.
But there is another problem with a code like the following one (let us assume a date format like MM/DD/YYYY and a 12 h time format like h:mm:ss.ff ap where ap is either AM or PM and ff are fractional seconds):
rem // Resolve AM/PM time:
set "HOUR=%TIME:~,2%"
if "%TIME:~-2%" == "PM" if %HOUR% lss 12 set /A "HOUR+=12"
if "%TIME:~-2%" == "AM" if %HOUR% equ 12 set /A "HOUR-=12"
rem // Left-zero-pad hour:
set "HOUR=0%HOUR%"
rem // Build and display date/time string:
echo %DATE:~-4,4%%DATE:~0,2%%DATE:~3,2%_%HOUR:~-2%%TIME:~3,2%%TIME:~6,2%
Each instance of %DATE% and %TIME% returns the date or time value present at the time of its expansion, therefore the first %DATE% or %TIME% expression might return a different value than the following ones (you can prove that when echoing a long string containing a huge amount of such, preferrably %TIME%, expressions).
You could improve the aforementioned code to hold a single instance of %DATE% and %TIME% like this:
rem // Store current date and time once in the same line:
set "CURRDATE=%DATE%" & set "CURRTIME=%TIME%"
rem // Resolve AM/PM time:
set "HOUR=%CURRTIME:~,2%"
if "%CURRTIME:~-2%" == "PM" if %HOUR% lss 12 set /A "HOUR+=12"
if "%CURRTIME:~-2%" == "AM" if %HOUR% equ 12 set /A "HOUR-=12"
rem // Left-zero-pad hour:
set "HOUR=0%HOUR%"
rem // Build and display date/time string:
echo %CURRDATE:~-4,4%%CURRDATE:~0,2%%CURRDATE:~3,2%_%HOUR:~-2%%CURRTIME:~3,2%%CURRTIME:~6,2%
But still, the returned values in %DATE% and %TIME% could reflect different days when executed at midnight.
The only way to have the same day in %CURRDATE% and %CURRTIME% is this:
rem // Store current date and time once in the same line:
set "CURRDATE=%DATE%" & set "CURRTIME=%TIME%"
rem // Resolve AM/PM time:
set "HOUR=%CURRTIME:~,2%"
if "%CURRTIME:~-2%" == "PM" if %HOUR% lss 12 set /A "HOUR+=12"
if "%CURRTIME:~-2%" == "AM" if %HOUR% equ 12 set /A "HOUR-=12"
rem // Fix date/time midnight discrepancy:
if not "%CURRDATE%" == "%DATE%" if %CURRTIME:~0,2% equ 0 set "CURRDATE=%DATE%"
rem // Left-zero-pad hour:
set "HOUR=0%HOUR%"
rem // Build and display date/time string:
echo %CURRDATE:~-4,4%%CURRDATE:~0,2%%CURRDATE:~3,2%_%HOUR:~-2%%CURRTIME:~3,2%%CURRTIME:~6,2%
Of course the occurrence of the described problem is quite improbable, but at one point it will happen and cause strange unexplainable failures.
The described problem cannot occur with the approaches based on the wmic command as described in the answer by user Stephan and in the answer by user PA., so I strongly recommend to go for one of them. The only disadvantage of wmic is that it is way slower.