I thought of posting this in the 1914 thread, but the apologists are already hijacking that.
When I was writing "Armageddon's Disciples" I wanted to set it within an apocalyptic religion struggling with failed prophecy. As the books evolved I kept updating the dates for that failed prophecy.
One early version went this way:
The trampling of the nations began in 617 BCE when Jehoiachin was taken captive to Babylon. That started the 1260 days (or years) which ended in 643 CE, this year being marked by construction of the Dome of the Rock. That started the 1290 day (or years) which ended in 1933 which was supposed to be the end of the world but turned out to be the year the Disciple religion started.
Over the years I tinkered with the numbers and eventually decided you could make anything sound prophetic.
Then I ran up against this quote from Mark Twain: “The only difference between reality and fiction is
that fiction needs to be credible.” A member of my writers group told me in dead seriousness, that readers simply would not believe people were dumb enough to follow a religion that predicted the end of the world thirty years ago. As published, I left out the calculation and just noted that the failed prophecy took place "a few years ago."