Nothing in the Bible says that the tribulation will, necessarily, start immediately after the Rapture. No one knows how much time will pass.

The Rapture is not the Second Coming. There is only one Second Coming. In the events of the Rapture, Christ comes into the atmosphere. As I said before, He does not actually touch down on earth.

"open the door to the great tribulation"
That is probably an accurate statement, but it still doesn't mean the tribulation will start immediately after the Rapture occurs.

During the tribulation, after the church has left the earth, anyone with eyes should be able to see that the tribulation is upon them, and then be able to start counting down, It will be that bad. But, 2 Thes. 2:11 says: And for this reason God will send upon them a deluding influence so that they might believe what is false. This deluding influence will no doubt prevent that.