There are different dating schemes for the canonical Gospels.
i) One common approach is to treat it like the domino effect. Assuming Markan priority, and the literary dependence of Matthew and Luke on Mark, if we have a date for Acts, then Luke's Gospel predates Acts, and Mark's Gospel predates Luke. So you work back from Acts as your chronological benchmark.
Likewise, it's sometimes argued that John's Gospel takes the Synoptic accounts for granted, making passing references that would be unintelligible apart from knowledge of those prior accounts ("undesigned coincidences").
According to one stock argument, Acts ends at that juncture because Luke takes the action up to the present. He says nothing more because nothing more had happened.
I think that's easily the most plausible explanation for the abrupt ending, which leaves things up in the air.
a) Why would Luke devote a quarter of his church history to Paul's arrest/trial/appeal, only to say nothing about the final disposition, if this was written after Paul's execution? That's quite a build-up. He knew his readers would be curious about the outcome.
b) Moreover, Luke already mentions the martyrdom of two Christian leaders (Stephen, James), so why would he avoid mentioning the martyrdom of his hero (Paul), if it was written after the fact?
ii) Given (i), scholars date Acts to 62-64. But that in turn pushes Luke further back in time, and Mark further back in relation to Luke.
iii) In addition, scholars like R. T. France have argued that Matthew refers to many topical Jewish customs and controversies which would be moot after the fall of Jerusalem. Would Matthew's selective account preserve information irrelevant to his audience?
iv) Admittedly, dating is less significant than authorship so long as a later date doesn't preclude traditional authorship.
However, a potential casualty of dating one or more Synoptic gospels to sometime after 70 AD is the argument from prophecy. In the nature of the case, prediction and fulfillment are more impressive if the prediction was publicly known in advance of the event. If, however, both the prediction and its fulfillment are recorded after the fact, then is that a prediction or retrodiction?
If all three Synoptic Gospels were written after the fall of Jerusalem, then the prophecy of its impending downfall loses some evidentiary value. That invites the charge of prophecy ex eventu.
There can, of course, be a significant gap between the time a prophecy is uttered and the time it's written down. The prophecy itself may be in circulation. Be remembered by the original audience who heard it, and, by word-of-mouth, for others who were not in attendance at the time. Oral tradition can precede a historical record. So it could still be common knowledge for people living before the event. In principle, they could vouch for the oracle. But, of course, later readers of the Gospel have only the Gospel accounts. We live on the other side of the event, looking back.
In principle, Mark could be pre-70 AD while Matthew and Luke could be post-70 AD. In one respect, their record of the prophecy would be dependent Mark insofar as they copied and edited Mark's version, rather than independent corroboration.
If, however, they lived before the event, then even though they only wrote after the fact, that would still constitute independent attestation. And, indeed, Matthew and Luke do use earlier sources. So we need to distinguish between the date of their Gospels and the date of their sources.
If, however, all three antedate 70 AD, then that simplifies the argument from prophecy. In that case we clearly have priory and multiple-attestation alike.
v) Here's a classic case for the earliest possible date of Mark:
vi) Some commentators object that it's a dubious inference because Mark also has an abrupt ending. That, however, is a poor comparison, for the ending of Mark is, itself, considered to be problematic, requiring a special explanation.
vii) Some scholars think Mk 13 reflects a Roman setting, with specific reference to the Neronian persecution of Christians, which would date Mk to the late 60s. However, that's been challenged:
The linguistic data, for example, have come under fire from Theissen (Gospels, 244-49) and Marcus ("Jewish War," 443-46), who have argued that in 12:42 and 15:16, Mark is not substituting western terms for eastern equivalents but explaining imprecise Greek words by means of precise Latin ones.
More importantly, although the persecution of the Roman Christians under Nero is the best-attested case of persecution of Christians in the first century, it is not the only such instance (for surveys, see Beare, "Persecution," and Potter, "Persecution"). Acts, the Pauline correspondence, and later church sources attest sporadic persecutions before Mark's time, in Judaea (Gal 1:13,22), particularly in Jerusalem (e.g. Acs 5:40; 7:54-8:3; 12:1-5; 21:27-36; 23:12-15; Josephus Ant. 20.22); in Damascus in Syria (2 Cor 11:32-33; Acts 9:1-2,23); in several cities in Asia Minor (Acts 13:50; 14:19; 19:24-34); and in Greece (Acts 16:19-24; 17:5-9,13; 18:12-13). Some of these persecutions seem to have been spontaneous acts of mob violence (cf. "hated by all" in Mk 13:13); Josephus, for example, mentions that James, the brother of Jesus and head of the Jerusalem church, was killed by a Jewish mob in 62 C.E. (Ant. 20:200), and in Acts the Christians' antagonists are often simply called "the Jews" (Acts 9:23; 12:3; 13:50; 14:19, etc.), though this term may sometimes denote Jewish authorities rather than the general populace (e.g. 13:50.
In any event, some of the actions against Christians involved rulers as well (cf. the reference to trials before kings and rules in Mk 13:9). In Acts 12:1-5, for example, we hear of the involvement of Agrippa I of Palestine, sometime before his death in 44 CE., in the execution of James the son of Zebedee and the incarceration of Peter in Jerusalem, and in 1 Cor 11:32-33 we are told of an attempted arrest of Paul by agents of the Nabataean King Aretas in Damascus. Official persecution of some sort is also implied…by Paul's arrests of Judaean Christians before his conversion (Acts 8:3), by the letters he obtained from the high priest in Jerusalem to authorize the arrest of Damascene Christians (Acts 9:1-2), and by the punishment meted out against him after his conversion by the magistrates of Derbe in Asia Minor (Acts 16:22-24). Examinations of Christians before rulers, moreover, are described as taking place in Corinth (Acts 18:12-17), Jerusalem (22:30-23:10), and Caesarea (23:33-26:32), and beatings in synagogues or other Jewish venues, similar to those mentioned in Mk 13:9, are referred to as occurring in Jerusalem (Acts 5:40) and unspecified locations (2 Cor 11:24). The persecutions described in Mark 13:9-13, therefore, do not necessarily point toward Rome or the events under Nero.
Indeed, it may even be questioned how well Mk 13 fits the circumstances of the Roman persecution described by Tacitus. If this chapter really reflected those events, in which Nero was such a dominating presence, would we not expect a Nero-like figure to be prominently featured in the Markan "prophecies"? Tacitus makes it clear that the persecution of 64 C.E. was instigated by Nero himself and that he played a central role in it, even using his private gardens for the slaughter of Christians. This information is intrinsically credible, since Nero would have had a plausible motive (scapegoating the Christians for a crime of which he himself was suspected) and since Tacitus himself had no love for the Christians and thus would probably not have invented the charge merely to slander Nero. But Mk 13 does not concentrate disproportionately on the wickedness of a Nero-like pagan king; there is only an incidental reference to hearings before rulers in 13:9, not the sort of preoccupation with regal wickedness that we see, for example, in the descriptions of the "beasts" in Dan 7 and Rev 13. And Nero is an unlikely candidate for the "abomination of desolation" in 13:14, since he never visited or even planned to visit Palestine, and the "abomination" is probably some sort of desecration of the Jerusalem temple. If Mk 13 really came out of the Neronian persecution, would we not expect it to focus more, as in Daniel and Revelation do, on a bestial, anti-God figure? Joel Marcus, Mark 1:8 (Yale U, 2002), 32-33.