cocoa : assign nstimeInterval value to nsdate?
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I want to assign nstimeInterval value to nsdate.

      NSDate *startTime=[NSDate date] ;
      NSDate *endTime=[NSDate date] ;
       NSTimeInterval difference=[endTime 
       timeIntervalSinceDate:startTime];

        startTime=difference; This gives me an error. 
how can i assign my difference result of nsTimeInterval type to nsdate?

thanks in Advance