Here's what I would do instead:
First, I would ensure each cell has a unique identifier of some sort.
Then, I would create an NSMutableArray or NSMutableDictionary as a property of your UIViewController that stores each cell's corresponding NSTimer.
Next, if a reference to the NSTimer from a table view cell is absolutely necessary, I can create a weak reference to the NSTimer in the dictionary.
Then, when I go to delete the table view cell, I can also invalidate the timer by looking it up in the NSArray/NSDictionary and then deleting it.