Horn’s Projection Early in his first book, Wormwood (p. 27), 

Horn provides the initial basis for his computation: Assuming for the moment that Apophis is biblical Wormwood and that 2029 would thus represent a period sometime around the middle of the great tribulation period when the trumpet judgments begin, THEN:

Monday, October 13, 2025 (April 13, 2029, minus three and a half years) would be the approximate start date of the dreaded seven years of tribulation foreseen in Scripture (see Matthew 24:21; Revelation 7:14; and Daniel 12:1). ….Depending on one’s particular position, this would place the last possible date for a pretribulation rapture happening sometime around October 13, 2025. 

 Horn rejects NASA’s (Never a Straight Answer) - near miss projection. Horn contends that if Apophis is coming close enough to knock out Earth’s satellites, as NASA predicts, then impact with Earth is inevitable, despite NASA’s denials, making the impact date of Apophis 4/13/2029