Rayford's calculations were based on two passage from the book of Daniel, in the Old Testament. One predicts a period of seven years between an agreement that results in sacrifices resuming in Jerusalem and the "consummation" (or end). The other predicts a period of just 2300 days from the time the sacrifices begin until the sanctuary is first desecrated and then cleansed.
The difference between these two figures (2520 days and 2300 days) is presumably the time it will take to construct the Temple. (See box below.)
Daniel 9:24-27 is so written that it could be referring to two parallel agreements, one between the prince of this world and his followers, and another between the Prince of peace and his followers. One results in a physical Temple being built, and the other results in a spiritual Body coming together miraculously.
But, in either case, something horrible happens in the middle of the last seven years, resulting in an "abominable desolation" being inflicted upon both "temples".