An Important Birthday by Jacob Pedersen

An Important Birthday by Jacob Pedersen
mindreading, mentalism,
This trick will make you look like the new Rain Man - or a great mindreader.
You will be able to guess the age and birthday of a total stranger.
There is some mathematics involved that is so easy that noone will figure out how you do it.
There is no fake calculator or toxic force involved. This is a complete different trick.

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