I'm reading a book by a Christian author which purports to illuminate some miraculous coincidence between blood moon tetrads and Jewish festivals. One thing that he does without any explanation is that he calculates exact dates of Jewish festivals before Gregorian reformation and before the 4th century, when fixation of Jewish leap years probably took place. Moreover, he goes on to calculate dates of Jewish festivals even BCE! The author seems to be using some online converters, possibly http://calcuworld.com/calendar-calculators/hebrew-calendar-converter/, which allows even BCE calculations.

So my question is: How to calculate a 2nd century Jewish festival date in both Jewish and Gregorian calendars beyond any doubt? Hebcal for example tells me that 17 April 162 CE is 15 Nissan 3922. Can I believe that? If not, why?

Thank you for answers. If there is an explanation already anywhere on the Internet, I do apologize, but I've been searching a lot and found nothing on this so far.

  • Relevant: What is the significance of a Blood Moon in Judaism? - includes explanation of why blood moons might overlap with Jewish festivals.
    – Rish
    Aug 25, 2017 at 8:06
  • The Gregorian calendar has changed over time. I don't think this would be possible to do. In addition, what about leap years?
    – ezra
    Aug 25, 2017 at 17:03
  • Calculations of Jewish calendar are vague (and non-consensual) at best even until the medieval period (vide Calendar & Community). Backwards extrapolations of the modern Hebrew calendar, classical lunar Hebrew calendar and (proleptic) Gregorian calendar are described in Calendrical Calculations.
    – Argon
    Aug 27, 2017 at 1:40

1 Answer 1


It is not possible to know exactly when a given date would have fallen before the calendar fixing. Each month would be 29 or 30 days depending on when the moon came out. With clouds, you could have a bunch of long months.

Therefore, without records of when the new month was declared each month, it is impossible.

The best you can get is an approximate, based on calculations with the current fixed calendar.

  • 1
    About how good an approximation can you get? Is there a known upper bound on the error?
    – Double AA
    Aug 25, 2017 at 13:46
  • @DoubleAA Since the tekufah and the positioning of the chagim had to be correct, the upper bound could be calculated. Also the astronomical conjunction and estimated time of first seeing could also be calculated. This should allow a decent estimate of the maximum difference. Of course this would not show the reference in the solar calendar in use at that time. Aug 25, 2017 at 16:08

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