calculating brit mila

Please don't be offended but I am mystified why you would need an option to calculate something so simple?

We are given the mitzvah of brit milah in Bereishit parasha Lech Lecha (Genesis 17:11-12) in which we commanded to perform brit milah on the eighth day.

As a general rule, in Eretz Yisrael, the brit milah is performed on the eighth day irrespective. Here in the UK, and I suspect many other countries outside Israel, the mohel will aim for the eighth day but it is very much subject to the health of the baby. For example, the brit milah will be postponed if the baby is very jaundiced or if there are other health issues.

With regard to calculating the eighth day, you count the day of the baby's birth as the first day. So if the baby was born on Sunday, the eighth day would be the following Sunday.

I hope this helps.