score:11
The difficulty in verifying the math and the calculations is that every prognostication has its own formula. In order to verify Harold Camping, for instance, you have to look at the passages he is pulling from.
For example, here is one (and I don't endorse it) showing how they arrive at their numbers.
Probably the most important calculations revolve around the "time, times, and half a time" in Revelation, and the vision of the 70 weeks in Daniel.
There is one mathematical construct in the Bible that bears the most weight, however, and that is Matthew 24:36:
But concerning that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only
Upvote:2
The general point is that there are different ways to try to figure it out. There are generally two assumptions made (three if you count the elephant in the closet of being able to calculate at all) and these are:
These are generally private interpretations (warned against by the apostles) and you can tell because there is no general system. For instance, they may count the number of words in the Codex Vaticanus, or the number of days from creation to Christ, or the number of days taken literally in Revelation. The second step is then to use the symbols in the prophecy to figure out where in that process we are.
The emergence of these predictions is sometimes sparked by the discovery of an old document or the advent of access to a system of counting something in the scripture more accurately.
That all being said -- you can easily find the calculations for a given prediction if you look it up!