The actual event that starts the coming Tribulation is noted in Scripture but not the actual day/hour that it is scheduled to begin. In fact, Jesus makes this very clear in His Olivet Discourse, that the actual times of certain upcoming events is not for us to know until they actually happen. There’s a very good reason for this too.

If God had specifically told us when, for instance, the Tribulation would actually begin (and again, we know the event that kicks it off, but not the date), then He would have also been telling Satan. There are many things that God has kept and does keep close to His chest throughout Scripture. This He does for several reasons. First, His desire is to strengthen our faith first and foremost. If we knew everything, what need would be there to exercise faith at all? Second, anything God tells us He also indirectly tells Satan.

If Satan knew when the Tribulation was going to start, he would not worry about having someone ready to go as the Antichrist in every generation. He would simply focus the bulk of his attention on when he knew the Tribulation would begin and work accordingly. The fact that Satan has not known and does not know when the Tribulation formally begins has meant that he has had to have someone he could tap in every generation since the time Jesus walked this earth.