If working out the hebrew date on which one was born am I correct that any time before Tzes Ha’cohavim would be considered the previous day I.e we consider Bein Hashemashos as part of the previous day?
This depends on the purpose of the question.
For purposes of bris milah, a baby born after Shekiyah (during bein ha-shmashos) is considered to have been born “the next day,” and his bris is deferred to eight days later.
The same is true for purposes of the bar mitzvah — for joining in a minyan and so on: 13 full years are required, and we would use the later date rather than the day before (see Shulchan Aruch Ha-Rav 53:13; Mishnah Berurah 55:42)
However, the boy must be stringent and is considered a gadol from the first of the two days, and therefore the celebration of the Bar Mitzvah should be made on the first day, when he enters the yoke of mitzvos (Shevet Ha-Levi Vol. 4, no. 26).
For purposes of a regular “birthday celebration,” or for giving charity, fasting, etc. (which some people do on their birthdays), the matter is a safek, meaning the birthday could be the first day, and could be the second, and therefore one can “celebrate” both days.