Probably not the right answer but... Infinity. Just because your not living doesn't mean it's not your birthday.
Hunter got it one. you are only born once, but when you have annual birthdays you are just celebrating it, not having it.
Pronounced as one letter, And written with three, Two letters there are, And two only in me. I'm double, I'm single, I'm black, blue, and gray, I'm read from both ends, And the same either way. What am I?