I said that in Biblical mathematics, 2=3, but this actually has more to do with cultural presuppositions and translation. More generally, the rule is that x=x-1 if you go from Hebrew to English, and x=x+1 if you go from English to Hebrew.
What I am saying is that the Bible uses inclusive counting as was common in the ancient world and as it is still used in many parts of the non-Western world today. We in the Western world use exclusive counting.
Let me start with an example from Act 7:8:
RSV: And so Abraham became the father of Isaac, and circumcised him on the eighth day.
NET: and so he became the father of Isaac and circumcised him when he was eight days old,
GNB: So Abraham circumcised Isaac a week after he was born
Of these translations, only GNB is correct in English. RSV is ambiguous, unclear and unnatural. If the boy is born on a Monday, he must be circumcised on a Monday, 7 days later. If he is a born on a Sabbath, he must be circumcised on a Sabbath. In the Bible the date of birth is counted as day 1, the following day as day 2, etc.
I discovered this by accident one day in a town in Africa, when I asked someone for directions to a specific bank. He told me: Turn left on the third street and you will see it. So, I proceeded to the third street and turned left. I went all the way down the street, but there was no bank. So I turned left at the other end and went up the second street, and sure enough, there was the bank. Then it dawned on me. We had been standing on a street corner, and to the local person the street we were standing at was the first street, but stupid as I was, I only started counting from the next street. To me, the street I was at was considered to be street zero.
This topic has been covered in the UBS publication The Bible Translator (TBT 30 : 340-343) and also in Notes on Translation (no. 108, 1985). The idea can be shown/proven from many passages in the Bible, but probably the most decisive is Lev 23:15-16:
NIV: From the day after the Sabbath, the day you brought the sheaf of the wave offering, count off seven full weeks. Count off fifty days up to the day after the seventh Sabbath.
The starting day is Sunday and the closing day is Sunday, 7 weeks later. This is a period of 49 days, but since you count the starting day as day 1, you reach to 50 when you arrive at the closing day.
This inclusive counting is always used and that is why on the third day corresponds to two days later in normal English (and Danish).
There is a more tricky expression, namely for three days and three nights. The problem is that when you count inclusively, say, three days from Friday to Sunday, there can only be two nights in between. So, we are dealing with a Hebrew idiom here that most people get very confused about, because a literal translation in English is utterly misleading. In Hebrew tradition, it is very important to say more or less the same thing twice. It sounds better to say three days and three nights than three days and two nights. Likewise, Moses was on the mountain for 40 days and 40 nights, just as it rained during the flood for 40 days and 40 nights, and Elijah travelled 40 days and 40 nights and Jesus was tempted in the wildernes for 40 days and 40 nights. To a modern, scientifically focused mind, it is difficult to understand that both of these expression refer to 40 days, day and night, or 40 days with the intervening nights or, to be exact: 40 days and 39 nights. No English Bible translation has as yet recognized and dealt with the problem, as far as I know. The whole expression is an idiom and must be understood as an idiom where the sum of the meaning of the words does not add up to the meaning of the idiom as a whole.
I think that is enough mathematics for now.