According to whose authority must the 360 day Hebrew lunar year be used to make this calculation? I am not saying that this is wrong, necessarily, but I am questioning the authority of the person who simply assumes this is true. The Jews calculate years of reign of their kings according to solar years of 365 days, not lunar years. We know this from biblical data. They did have a 12-month 30-day religious calendar year, but every few years they would add an extra intercalary month, because the Jews were well aware that the year is more like 365 than 360 days long. Over the long run, Jews have calculated overall elapsed time using the solar year, not lunar. This is a fact. Right now is year 5781 in the Jewish calendar and, just so you know, those are solar, not lunar years. The fact is that the Jews use 360 day lunar years and 365 day solar years, depending on the context. So, which did God use in Daniel 9:24-25? The humble believer will say that he or she is not absolutely certain. We can let history help us to answer this question, rather than make a presupposition.
My guess is that you read a commentator who declared as fact that the 70 weeks are 490 years of 360 days. Is that what is said in Daniel? Does the author offer proof that the prophet is using 360 day years? Is he willing to concede that the Jews themselves used solar years to count the years of reign of their kings?
Just so you know, some authors do use 360 day years, but then they use the decree of Artaxerxes in 444 BC as the starting point of the 70 weeks. What is so surprising is that one arrives at the same date whether one uses the decree of Artaxerxes in Ezra 7 in his seventh year and then calculate using solar years, or if one uses the permission that the same king gave in his 20th year to Nehemiah in Nehemiah chapter 1 (which was not exactly a decree, but…) and 360 day lunar years, either way, one arrives at a date of 33AD for the end of the 70 weeks, if we assume the 490 days are years. Bottom line, there are two options and both work. How amazing is that?
Which is the correct one? I do not know. What I can say is that there are four options.
1. 7th year of Artaxerxes, solar years
2. 7th year of Artaxerxes, lunar years
3. 20th year of Artaxerxes, solar years
4. 20th year of Artexerxes, lunar years.
Of the four, either #1 or #4 fits the facts of the death of Jesus during the 70th week and #2,3 do not. I reject #2,3, but this is because of a bias/presupposition on my part that the Bible is inspired by God. I openly admit my bias here. Is this circular reasoning, perhaps, but the fact that any of these works at all is further evidence for the inspiration of the Bible to me!