Whether it's your birthday or mine, there are only 365 different possibilities in a year. A single human could go through all those possibilities in a few hours. A computer in a few milliseconds. Even if you add the year to the birthday, we're only talking seconds or minutes for a computer to guess each possibility. You have to remember that computer crackers are not specifically targeting YOU usually, they are targeting anything they can get. If you happen to be the poor soul who's birthday is January 1st, 2001 and you use 010101 as your password because you think there are a lot of birthdays out there - too many for even a computer to figure out - you're going to be owned on about the third guess.