"...the fifteenth year of Tiberius Caesar (Luke 3.1), reckoned as 27 or 28 CE. The date of his crucifixion is debated, with 14th Nissan 30 or 33 the chief alternatives, and the former gaining more support. The former would also fit with the general impression that Jesus' mission must have extended over two or three years, given particularly the Fourth Gospel's mention of three Passovers (John 2.13; 6.4; 11.55). Beyond that, the discussion quickly becomes bogged down, with the data affording no firm ground on which to advance..."Now, why is this a nearly perfect summary? He left out 29 AD. In strict Roman chronology, the debate on Tiberius' 15th year should be 28 or 29, not 28 or 27. Tacitus himself would pick 29 but we may stretch Luke inclusively to 28. However, the wishful stretching required to suggest 27 [the so-called 'co-emperor' dating] comes in only because of a need to save the traditional preference for 30 over 33. (Harold Hoehner was one of those who got this right, though he got other things wrong.) This preference also limits the duration of Christ's ministry to 3 years at most.
It is extremely telling (of the scholarship which is being summarized) that Dunn mentions only John's Passovers in relating the duration of ministry. Dunn implies one additional passover of Matthew, Mark & Luke when he says "or three years", but the "or" makes me certain that the unfirm bog he immediately mentions is, in his summary, the synoptic "problem". Generally, the academy seems willing to discuss a bit of chronological data from the synoptics if all three agree on it - such as the deutero sabbath and Good Friday. But assumptions of source theory (evidently) are upheld to inhibit any critical attempt to reconstruct one sequence of gospel events for the whole of Christ's ministry. Well...
This is the point where I like to chime in. Simply put, it is not enough to say we cannot proceed. With an alternative source theory (fair's fair, after all) we can easily separate questionably conflated events. Assume for the sake of argument that three pairs of similar seeming events were actually six separate events, and the synoptic/Johnine chronology slides into an exceptional cohesion. This may or may not *prove* itself to be solid ground, but at least the assumption is academically valid ground from which to proceed. Surely such a conditional, potentially reliable reconstruction is far preferable to a deliberate, permanent head-in-the-ground condition. (Is it not?)
Just like the forest is more than the trees, I believe a full reconstruction of scriptural events is more valuable than interpreting difficult scattered verses. Accepting Johnston Cheney's blended chronology of all four gospels settles a four year duration [bookended by five passovers] for Christ's ministry, which eliminates 30 AD in favor of 33 and thus settles 28 as the year Luke must have meant as Tiberius' fifteenth. In all of this, urges to defend individual scriptures must wait until our last priority. If our faith in the scriptures is indeed justified, then delaying their defense will bring no harm, and could instead bring surprising and much needed perspective.
Mathematicians know this: sometimes the correct solution to a problem can only be found by working out all potential options, although each one is grounded by an assumption or logical leap. In fortunate cases, all but one option eliminate themselves from contention. Or, to put that another way... Sometimes you gotta put your foot in the river, before the waters can part.