Question

If you have lived for a million seconds, how many birthdays have you had?

Answer 1

You will only be 11.57 days old. 
Arguably, however old we are, we only have one 'birthday' which is the day of our birth. What is referred to as our first birthday is actually the the first anniversary of our birth and subsequent ones are the relevant anniversaries our birth. 

Answer 2

A million seconds is not that long.  My next birthday is in  1,296,000  seconds
from now, and I have already lived more than  2.3 billion seconds !

(1,000,000 seconds) x (1 day / 86,400 seconds) =

                 11days 13hours 46minutes 40seconds .

It's possible that you might have one birthday within that length of time,
but it's not guaranteed.  It would have to be the RIGHT million seconds.

If that's been your whole life so far ... you are 1 million seconds old ..,
then you have not had your first birthday yet.