Birth day is easy, anything under 22 people is typically under 50/50 and of course 1 in 365 chance.

 First person could have any day of the year: (p = 365÷365 = 1)

 The second could have any of the other 364 birthdays: (p = 364÷365)

 Multiply them and you have about 0.9973 as the probability that those two have different birthdays, or 1−0.9973 = 0.0027 as the probability that they have the same birthday.

 A simpler equations that anything under 20 people is 0.27% for birth day.


 Calculating the birth year will take some time as the collective number of births globally over the year 2003 would have to be agreed upon, and then whatever parameter of total years you are willing to accept.

 For instance if you want the probability that any two people were born on the same day and year since the known existence of man until today would take some substantial research.

 What period of time do you think Humans have existed?

Yea. One killed two, the other will kill tens of thousands.