So one night I decide to try to find my Hebrew birthday.
And I go to http://www.chabad.org/calendar/birthday.asp?AID=6228 and plug in the information, right?
Turns out I had to be difficult and be born in between sunset and nightfall, so the date I was born on is questionable.
It’s either 27 Elul or 28.
If it helps, on the day I was born, sunset began at 6:55PM. According to this site, the beginning of nightfall was 7:37PM, and I was born at 6:59.
So which day would be my Hebrew birthday? When does the day offically change on the calendar: the minute the sun STARTS to go down, or the minute it’s officially gone from the horizon?
Hey Zev Steinhardt, get in here! 
I’m not a Rabbi, but here goes…
There is a rabbinic dispute as to when the day begins. One opinion is that the day begins at sunset, the other is that it begins at nightfall.
As a result of this dispute, we generally go according to both. So, for example, the Sabbath begins at sunset on Friday night, but doesn’t end until Saturday night at nightfall. This way, the Sabbath is not violated either way.
The period between sunset and nightfall is called bein ha-shmashos. In practice, therefore you could celebrate either day. Once you are bar mitzvah, there is no way that it makes a halachic difference.
Zev Steinhardt
