I cannot speak for anyone else but for me it is a relatively simple process. The year is based on how old I need for the character to be. However the month and day is simply the date of character conception. Now that is not the date that the character is approved or even submitted. It is the date that I first put pen to paper (or finger to keyboard) and starting working on the character. Usually my PC/PNPC characters take time to build. So I look back at my original notes and that for me is the date of birth.
My 2 credit chips!