The Death Clock site predicts how long you will live given your gender, date of birth, smoking status, BMI (this will be calculated for you given your weight and height) and degree of optimism/pessimism.
I got this from from Cordeloia Fine, A Mind of its Own, which, among other things, discusses how the brain 'protects us' with vanity. The claim is that with respect to expected lifetime, we all believe we will live longer than we are likely to. This is similar to the view that we all believe we are better drivers than the next guy, but more specifically it relates to work in terror management theory showing how we deal with the prospect of inevitable death. According to Pyszczynski et al. (2004) a healthy brain is:
'...a protective shield designed to control the potential for terror than results from awareness of the horrifying possibility that we humans are merely transcient animals groping to survive in a meaningly universe, destined only to die and decay'.
My instincts are to claim that the death clock absurdly overpredicts the pace of my approach towards boot hill - so my own reactions accord with the theory. Note however that the Death Clock forecasts are not nationality-specific and place a lot of weight on BMI - moderate degrees of being overweight penalise your life expectancy strongly. As I have argued elsewhere this link is by no means firmly established. The Death Clock site carries weight-control advertising which raises suspicions.
I do however recommend Cordelia Fine's book - it is an interesting, non-technical read- she is at the University of Melbourne's Centre of Applied Philosophy and Public Ethics.