There's a very simple definition of julian dates: 1st of January 4713 BC at 12:00:00 GMT (Greenwich) is Day 0.
Julian dates are merely incremented by 1 for each day at noon GMT. As for fractions, a day is divided into 1/24's. So 1 hour is 0.041666667, 12 hours is 0.5 etc. Couldn't be simpler.
Try inputting 1473 BCE, Jan. 1st into the calculator at the link above.
(notice: the definition is simple .. the calculations are another matter
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