title: timeIntervalSinceDate gives exc_bad_access or autorelease errors
body: <p>So I have an object which represents a line drawn on the iPhone screen.  I also have a ball moving around on the screen, and when the ball and the line intersect, the string is given a life.  For my current build I am receiving this error constantly, with no crashes, but the ball stops moving on the screen:</p>
  
  <p>2011-08-18 11:00:05.436 myProgram[192:5e03] <em>*</em> __NSAutoreleaseNoPool(): Object 0x1531c0 of class NSCFString autoreleased with no pool in place - just leaking</p>
  
  <p><strong>Line.h:</strong>
      @interface Line : NSObject {</p>
  
  <pre><code>//time properties
  NSTimeInterval life;
  NSDate *startTime;
  NSDate *currentTime;
  }
  
  
  
  //time properties
  @property (nonatomic, retain) NSDate *startTime;
  
  
  -(void) updateLife;
  
  -(void) beginLifeTracking;
  
  @end
  </code></pre>
  
  <p><strong>Line.m</strong></p>
  
  <pre><code>@synthesize startTime;
  
  -(void)beginLifeTracking {
  [self.startTime release]; //not sure if self is necessary here but startTime is released
                            //in case the same string is hit again
  self.startTime = [NSDate date];
  startTime = [NSDate dateWithTimeIntervalSince1970:0];
  //NSLog(@"Time Interval: %f",startTime);
  }
  
  
  -(void) updateLife {
  currentTime = [[NSDate date] retain];
  NSLog(@"breakpoint1");   
  life = [currentTime timeIntervalSinceDate:self.startTime];
  [currentTime release];
  }  
  </code></pre>
  
  <p>I'm assuming this is some sort of memory management error, but all my attempts to remedy it have failed.  I'd really appreciate an explanation of what I'm doing wrong here. Thanks!</p>