.bat file scripting

Hey, I've been working on a .bat file that evaluates an expression  the user enters into the program. The problem i am having is when I enter an expression into the program it wont display the answer.
Here's my code:
@echo off
Title Calculator
Echo Welcome To Command Prompt Calculator
Echo Copyright Eric Bracht 2008
pause
:A
setlocal
cls
Echo The Time is
Time /t
Echo The date is
Date /t
Set /p C=Calculate:
cls
Set /a %C%
If errorlevel 1 echo Error (level 1)
If errorlevel 2 echo Error (level 2)
pause
goto A
Thanks

>>14
I don't :(