The stupid part about the old understanding is the claim that the preaching work will end before the great tribulation starts. Matthew 24 clearly indicates that the preaching work is the the last thing to be accomplished before the end comes.
As far as I know, the GB has always used the term 'the last days', to describe the last hundred years or so, without ever using the term ' the great tribulation'. By their own calculations and description of the world, shouldn't we be living in the great tribulation right now? I mean, everything fits- just like at almost any other time in history.