Heard it all before, mate. People who know precisely when our Lord will return (hereinafter, "The Enlightened") are two a penny. They all proclaim that they've got it all figured out where everyone else has missed it, usually based on their vastly superior knowledge of Greek (often via Strong's), their unequaled analytical skillz, and the truly invaluable aid of the Holy Spirit. They're always opposed by "scoffers", the spiritually blind, "foolish virgins", etc. If they're successful at all, they acquire a stalwart group of supporters who have the necessary vision and who are spiritual enough to See The Truth and accept what The Enlightened is peddling at face value.
The tricky part for The Enlightened is making sure that their revelation of the date for The Rapture is scheduled close enough to keep the Stalwarts' enthusiasm up, but not so soon that the the date comes and goes without incident. Then damage control becomes necessary to prevent losing all the casual supporters and most of the Stalwarts. Just as nothing succeeds like success, nothing fails quite as emphatically as failure, especially in the End Times prognostication business. When a date fails, as they all ultimately do, immediate action is required to keep everthing from coming apart like a two dollar watch.
The standard damage control technique is to explain that the Enlightened simply missed some of his calculations, and that he failed to account for the differences in calendars, some critical verb tenses in the Greek/Hebrew/Syriac/Latin, didn't take into consideration the procession of the equinoxes, the Doppler Effect, and/or the 14th Amendment. They then gen up a new date and time, this one an absolute lock. But they only get one shot at that; if The Rapture doesn't happen on the new date (spoiler alert - it won't), even most of the remaining Stalwarts will head for the exits. The practice at that point is to "spiritualize" the whole thing, declare that the Lord actually did do something or the other on the date predicted, but that only the unco spiritual were able to discern it. This will work to some extent, and in the past has resulted in the creation by the few remaining Stalwarts of new denominations, notably the Jehovah's Witnesses and the Seventh Day Adventists. From a practical standpoint, though, after the second miss the Enlightened is all done as a spiritual whatever and it's time to go back to the day job.