Some people believe the great tribulation occurred in the first century a.d.
Jesus said that the prophecies of the end times would be fulfilled before the last of the generation, and they were. For some reason many Christians are completely blind to the great prophecies of the Lord. The present era is supposed to be that of the increase of the kingdom of God somewhat indefinitely.
People have a difficult time with the Revelation. Martin Luther said it had a lot of straw. Some have made a career of interpreting the book as a great future fantasy. Ancients might refer to a comet as a dragon with a tail for example so modern take the idea of a space dragon literally seeking to eat the child of the women who fled to the desert. John had to conceal messages to the faithful from Nero, so his language was coded a little apparently.
Garrison Gibson: books and blog 