You would have to talk to the scholars that study them. Hmm, let me see. I may be back.
Okay, I am back. Here is a link that explains how they got those dates:
https://bibleoutsidethebox.blog/2017/07/24/when-were-the-gospels-written-and-how-can-we-know/
A small excerpt:
What’s war got to do with it?
As we all know, the Gospels tell us that Jesus was crucified in Judea at around 30-33 CE. This means that the Gospels were necessarily composed after c. 33 CE. But how long after? When it comes to evidence pertinent to establishing a lower date limit for the Gospels, the most glaring evidence would be that the Gospels make explicit reference to the Roman-Jewish War (66 – 73CE). Specifically, the Gospels refer to when Roman soldiers surrounded Jerusalem in 67 CE, and most notably the Gospels mention the complete destruction of the Jerusalem Temple, which happened in 70 CE (see e.g.,
Luke 21 and
Mark 13). According to this scholarship, the Gospels were in all likelihood written after these events since they make direct mention of them. Nevertheless, those who espouse early composition dates for the Gospels ordinarily assert that the sacking of Jerusalem and destruction of the Temple as mentioned in the Gospels constitutes prophecy, and thus, the Gospels
must have been written
prior to these events – lest these prophecies attributed to Jesus be rendered as
prophecies after-the-fact.
Setting aside for now the circularity of that view, to insist that the mention of the siege and Temple destruction within the Gospels be taken
only as evidence of prophecy and
not as evidence that the gospel writers had
historical knowledge of these events amounts to a methodological double standard – especially when we are not equally charitable with secular/pagan writings attesting to would-be prophecies. If we’re going to appeal to customary academic considerations in our historiographic analysis of texts, then we have to be consistent in our methodology when we do so. We cannot make arbitrary exceptions.