An Important Birthday by Jacob Pedersen

This trick will make you look like the new Rain Man
You will be able to guess the age and birthday of a total stranger.
There are some mathematics involved that is so easy that most people will not figure out how you do it.
There is no fake calculator or toxic force involved. This is a completely different trick.

You can do this totally imprompty. It only demands a stranger/spectator with their own phone (which you never touch).

(It may be best combined with other tricks)