A couple of years ago, I wrote a post about internal and external evidence that Acts was written in the early to mid sixties. In a Facebook post earlier this year, I wrote about some other evidence for Acts' earliness and reliability. What I want to do here is make some other points to supplement the previous two posts.
The dating of Acts is important not just because of the implications the dating has for Acts, but also because of the implications for the dating of the gospels. Luke was written prior to Acts. Most scholars think Luke used Mark as a source, so dating Luke earlier has implications for Mark. Even if Luke didn't use Mark as a source, the similarities between Mark and Luke, as well as their similarities with Matthew, are best explained if the three documents were written within months or years of each other rather than a decade or more apart. So, an earlier date for Luke would imply earlier dates for Mark and Matthew as well, even if none of the gospels used another one as a source.
Proponents of a later dating of Acts often appeal to Acts 1:8 to explain why the book ends where it does. My first post linked above discusses some of the problems with that approach.
Another factor that should be taken into account is how little effort Luke makes to parallel 1:8 and the ending of the book, even though paralleling the two would have been so easy. For those who think Luke would have been willing to alter what Jesus said in 1:8, why didn't Luke add a reference to Rome? More significantly, why doesn't 28:30-1 say anything to the effect that the people Paul was teaching took the gospel to the ends of the earth? Luke does make such comments about the individuals in 8:4 and 11:19. If 1:8 was as important to Luke as proponents of a later dating of Acts suggest, then why doesn't he parallel the language of that passage in 28:30-1? Instead, he just refers to how people came to Paul and were taught by him, with no mention of how they or Paul reached the ends of the earth. It's striking that Luke seems so uninterested in paralleling 1:8 and 28:30-1, even though he so easily could have made them more parallel.
By contrast, my view advocating an earlier date for Acts has Luke explicitly telling his readers that he's addressing "the things accomplished among us" (Luke 1:1), both what Jesus initially did in the world during his life on earth and what he later accomplished through the church (Acts 1:1). Since Luke was writing a history of Christianity up to his day, he had no need to identify his reason for ending Acts where he did by paralleling the end of chapter 28 to an earlier passage or anything like that. His reason for ending where he did would have been obvious. On the other hand, if Luke was writing long after the events of chapter 28, it's unlikely that he would have been so unconcerned about clarifying why he ended his work where he did.
In a previous post on the dating of Acts, I briefly discussed why it's problematic to claim that Luke ended the book where he did because he wanted to end with Paul and/or the gospel getting to Rome. I want to expand on those earlier comments. The arrival in Rome occurs in 28:14. If Luke wanted to end with Paul and/or the gospel getting to Rome, he could have just placed a summary statement, perhaps one paralleling 1:8 and similar to what we see in 8:4 and 11:19, at the end of 28:14. Instead, Luke goes on to describe what happened three days later (28:17), what happened on some day after that (28:23), then what occurred during two years that followed (28:30). He narrates a series of events that took place over years of time after the arrival in Rome. He isn't just trying to get Paul and/or the gospel to Rome.
Even if we were to grant, for the sake of argument, that 1:8 defines the scope of Luke's work, we'd still have to ask why he ends the book where he does within that scope. An earlier date for Acts would still make more sense, even if we accepted the interpretation of 1:8 that's so popular among proponents of a later date.
Some commentators say that Luke probably ended Acts where he did in order to give the book a happy ending. Craig Keener, for example, mentions that happy endings were more appealing to readers. I agree that happy endings were (and are) preferred by readers, and I suspect that Luke wanted his work to have one. But if he could make a happy ending out of a situation in which Paul was in Roman custody, awaiting trial, and his message had been rejected by so many of the Jewish people he encountered in Rome, Luke could have done the same with Paul's martyrdom, the destruction of Jerusalem, or some other later situation. Think of Stephen's martyrdom in chapter 7, for example. It's just after that martyrdom, a negative event in some ways when considered by itself, that Luke refers to how the Christian message was spread by those who were scattered abroad in the persecution (8:4). In much the same way, Luke could have ended Acts with something like Paul's martyrdom (or Peter's, since Peter, unlike Paul, was one of the apostles addressed in 1:8), followed by comments on how the Christian message was then taken to the ends of the earth by those Paul (or Peter) taught. Or, if Luke wanted something more positive than Paul's martyrdom, the evidence suggests that Paul traveled further after being released from Roman custody. Why not end Acts with Paul's journey to Spain, which would not only avoid the negative context of his martyrdom, but would also avoid the negative context of chapter 28 and would be more of a fulfillment of 1:8, since Spain was more representative of the ends of the earth for Luke than Rome was?
Think about how problematic the arguments are for a later date for Acts. We have to reject the most natural reading of the openings of both Luke and Acts, look to Acts 1:8 instead to find the scope of Luke's work, then see that passage fulfilled by somebody Jesus wasn't directly addressing there (Paul) in a region of the world that was far from the ends of the earth (Rome), even though Luke knew of individuals and regions in later history that would align better with 1:8. Even though he just wanted to get Paul and/or the gospel to Rome, Luke kept narrating years' worth of events after the arrival there. It would have been easy for him to have explicitly paralleled the end of chapter 28 to 1:8 (e.g., by making comments like he does in 8:4 and 11:19), but he didn't. Then he was misunderstood by his earliest interpreters, who thought Luke and Acts were written earlier rather than later.
Why is that sort of scenario supposed to be preferable to an earlier dating of Acts? Whatever problems there are with dating the book early, those problems are less substantial than the ones created by a later date.