So many assumptions! Who says there would be a complicated calculation for Christians to work out when Jesus was returning? I'll tell you who it was; it was those idiots who imagined that the Bible was sacred and unable to err and said that God would be unkind if he didn't leave a clue in the Bible. Thank you very much Charles Taze Russell and company!
But what did Jesus say with reference to the signs of the times? Mark 13,31 "I tell you this, the present generation will live to see it all". NEB (I threw away my NWT)
Jesus and therefore the Bible got it completely wrong. The Jesus story was another in a long line of false prophecies playing on the tender hopes of the desperately poor two millennia ago.
There was no get out clause like the Watchtower make out to say that if it didn't come in Jesus' generation it will come when the WTBTS says so. No! there is no mention of such a possibility. The Bible is rubbish for prophecy and is no yardstick for truth of any sort.
What it is good for however, is for beating the living daylights out of people and getting them to jump through hoops and making them believe that fairytales are real.