It's obvious that the account was referring to events that would happen around the time of the destruction of the temple - within "[that] generation" just as Jesus said. The Great Tribulation can have only one fulfilment because it is described as tribulation 'such as has not occurred before, nor will occur again.'
Another thing: the wars, food shortages, earthquakes - these are not part of a composite sign. Jesus was actually telling his followers to not view them as a sign - that they do not mean the end is imminent. Watchtower is actually doing exactly what Jesus was telling his followers not to do!
In this same synoptic gospel account, Jesus warns his followers against believing and following after false prophets who would come in his name and claim the time is at hand. Think about this: If the food shortages, earthquakes, wars, etc were truly a sign that the time is at hand then why are those prophets wrong for making such proclamations? Why is Jesus discrediting them? Answer: Because both the false prophets and the bad world conditions are wrong in their message that the end is at hand. Jesus was telling his follower: 'don't be misled by world conditions and don't be misled by false prophets'.