“...contrivance.... blah blah...”
Jews are quite certain of the calendar from the first day of creation until today, the 10th day of the 4th month in the 5773rd day since creation, actually. Contrive other reasons if you can’t resolve your calendar.
>> “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.