Why in heaven's name (I mean that quite literally!) do so many Christians then spend vast amounts of time, inordinate amounts of energy, immoderate amounts of speculation, asking precisely that very question? We have been asked to be "his witnesses" to the world, not his calculators for his return. It remains a thorough mystery to me why this is so, and has been so throughout Christian history.
But I suppose I do know the answer. It is far safer, far less demanding, to be a speculator than a witness. Speculators write books of calculations, hold seminars that attract thousands, rake in untold piles of loot, while prognosticating a certain time for Jesus' return. Witnesses, on the other hand, just witness to the truth of the gospel: the truth of justice for the whole world, the love of enemies, and the care for the marginalized and outcast. As Acts 1 makes so clear, the world needs far fewer speculators and far more witnesses.