How did early Gospel audiences recognize the narrative's temporal setting? The way it differs from one Gospel to another probably reflects something about the time of each Gospel's composition.
Luke peppers the opening chapters of his Gospel with explicit historical name dropping and Matthew follows his modified genealogy with a story about Herod to Archelaus but Mark's opening relies entirely on John the Baptist, even narrating John's arrest in the passive voice, with no mention of Antipas. Other than John (and Jesus himself), Mark's first explicit reference to anything specifically indicating that bygone era is "the Herodians" (3:6). The lack of any effort to explain either who they were or why they were so named probably suggests (1) that oral tradition in those days was strongly imbued with these basic elements of historical context, but also (2) that popular memory of the baptizer and his fate was so strongly tied to Antipas that mentioning John was chronologically explicit all by itself. Likewise, Pilate is introduced (15:1) with no explanation whatsoever. The overall pattern probably serves as yet another indication that Mark's Gospel was indeed written earlier than its cannonical siblings.
By the time of the fourth Gospel, the baptizer is chronologized not by his preaching and arrest but by his relationship to the Light. "The word dwelt among us and we have seen him... John bore witness to him and we all received his grace" is obviously rich with theological meaning but as a narrative opening it also reads like "back in the good old days, when our movement was just getting started." Apparently the younger generation of Christians could accept the story of John and Jesus as its own historical epoch, contextualized by itself.
The next chronological reference ("forty-six years" in John 2:20) is probably (per me) an allusion to the coming Jubilee, though whether this means the symbolic Jubilee of Jesus's death and resurrection and/or the literal Jubilee (and that whether per Wacholder or Zuckerman) is debatable. Following that, John's arrest (3:24) is again referenced using a passive verb. That Antipas is never mentioned may largely result from the beloved disciple's well known fixation on Jerusalem, but it also fits the the pattern of focusing on concrete details only when they are deeply meaningful, and preferably obscure (e.g., the wedding at Cana, the pool of Bethesda). But Pilate is identified as Judea's governor only because he is the named person who comes out of the governor's house (18:28-9), apparently indicating that Pilate was already somewhat well known to the earliest audiences of the fourth Gospel. Presuming Antipas was equally familiar to the audience (cf. 3:24 again), the overall pattern of usage probably suggests that christians had grown familiar with the historical contours of Jesus's public era, certainly from oral tradition but probably also from earlier Gospel texts.
All this fascinates me for its own sake, but on the whole it also seems that each era of Gospel writing was conditioned by its own temporal distance from the Jesus movement, resulting in each author sensing the need to engage different levels of awareness when communicating with their audiences.
These are just a few observations and thoughts that I had today while I was writing/editing a passage about audience and chronology.