>> “Jews are quite certain of the calendar from the first day of creation until today” <<
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Absolutely false!
There is over 200 years of injected error in the current Jewish calendar. That can be proven by simply comparing historically recorded astronomical events with the present configuration. Perhaps you think computers are unreliable, but I seldom (never) catch mine screwing up.
We Jews are certain... you can twist all you want. There are Jews alive today with family trees to King David. Our tradition is a bit more extensive than yours.