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.