Programmer - Weird behavior calculating timezone difference

I can't explain this. I have the following:

     $time += $res['timezone']; (The array equates to -5*3600 (EST))
     return gmstrftime('%c',$time);

When I echo $res['timezone'], I get "-5*3600" which is correct. When I put the array value in front of the time variable, I get the incorrect time. If I comment out the array value and replace it with -5*3600, I get the correct result. Why??