I realized that I misunderstood this comment (I assumed same fortnight meant "same fortnight number in a year" - when it should be same 14 day span) but I thoug

I realized that I misunderstood this comment (I assumed same fortnight meant "same fortnight number in a year" - when it should be same 14 day span) but I thought it was worth calculating this example anyway. The oldest person alive was born in 1903. Since then 3035 fortnights have passed. Assuming an even distribution of birth fortnights, there'd be a 3.4058% chance of two people having the same birth fortnight (1 - (P(3035,15)) / (3035^15), where P is for Permutations). It's still fairly likely, but far from the ~1 in 10^23 odds of having NO shared birthdays between 180 people.