I was born on sep 26 1991 and in the family there’s a big debate as to if we go by shkiah or tseis hakochavim bec I was born 2 min after shkiah that year so is it the 18th of Tishrie or the 19th of Tishrie?
If the Rav could give me a source if possible
It is really a moot point, because at this point it really doesn’t make any difference, because once you are past your bar/bas mitzvah your exact birthday isn’t going to matter anymore. The time period between shliya and tzeis is a “twilight” zone, and it isn’t clear if it is day or night. Therefore the poskim, regarding such a boy at his bar mitzvah, would be machmir not to consider his birthday until the second day, however for some things there is room to be lenient. ( See Halacha Berura vol. 4 Otzros Dovid 7).