Eschatology is the study of end times (future events). 

Mark 13:32-33

“But of that day and hour no one knows, not even the angels in heaven, nor the son, but only the Father. Take heed, watch and pray; for you do not know when the time is.”

1914

The watchtower (Jehovah’s Witnesses) predicted that the Battle of Armageddon would end in 1914. How did they come to this? They believe that the Great Pyramid of Egypt was inspired of God, just like the bible. They translate inches of pyramid measurements into calendar years, in attempts to tell the future. Evangelical Christians have also bought into this, as you will see with the sensationalism behind the 2012 date. The 2nd false prophecy of the Watchtower was in 1925, as this would mark the bodily return of Abraham, Isaac, Jacob and faithful prophets of old. The 3rd false prophecy of the Watchtower was in 1975. [ 1 ] 

1988

Matt 24:34

Truly I say to you, this generation will not pass away until all these things take place.

One view of this passage would be the most natural reading; the generation who he was talking to directly. The second view is a generation in the future. Hal Lindsey believed "This generation" referred to the generation of 1948. A generation in the bible is about 40 years. So add 1948 + 40 and you get 1988. Hal believed Jesus would return in 1988. 

1994

Harold Camping predicted Christ would return on September 6. 1994

2011

Harold Camping predicted Christ would return on May 21, 2011. 

How can you believe Christ has already returned if the gospel hasn't been preached to the whole world ?

Matthew 24:14 (AD 50) 

This gospel of the kingdom shall be preached in the whole world as a testimony to all the nations, and then the end will come.

The question is  being asked from a futurist view. The historical view is that  the end  is not referring to the end of the cosmos, but rather the end of the Old Testament Age. Paul proclaimed that the gospel had already been preached to the whole world. This refers to the known world at that time.  

Colossians 1:5-6

because of the hope reserved for you in heaven, of which you previously heard in the word of truth, the gospel which has come to you, just as in all the world also it is bearing fruit and increasing

Colossians 1:23  (Ad 60-62)

 if indeed you continue in the faith firmly established and steadfast, and not shifting from the hope of the gospel that you have heard, which was proclaimed in all creation under heaven, and of which I, Paul, was made a minister.

Video - Historic Christianity Vs. Scripture? #01: Gospel Into All the World? - Don K Preston   View 1 

Video - Has the Gospel Been Preached to the Whole World? - Michael Brown   View 2 

Cited Resources : Jehovah’s Witnesses Answered by David Reed [ 1 ] ,