Chronology of the life of Christ
The Year of the Birth of Christ - late 6 - mid 5 BC
The modern calendar counts years from the birth of Jesus. But Jesus' birth-year was miscalculated to 1BC/1AD. This was probably because of the statements in Luke that John the Baptist began preaching, with Jesus shortly after, in the 15th year of Tiberius (28-29 AD) and that Jesus was around 30 when he did (Luke 3:23). Jesus' birthdate is neither 1 BC nor 1 AD (nor the non-existent 0 AD), but is in fact a few years before 1 AD. The most common date given for the year of Jesus' nativity is 4BC. This is because of the mention of his birth while Herod was alive (Matthew 2:1), sometime shortly before his death (Matthew 2:19; in v.20 Jesus is still presumably an infant).
We can see from the statement in Matthew 2:7, 16 that it is said that Jesus was born at least a year and some months (less than 2 years according to v.16) prior to the Magi's visit. It's not said how long after the journey to Egypt Herod died, but the text implies that it wasn't too long; perhaps a few months. In conclusion, we can assume Jesus was around 1.5-2 years old at the time of Herod's death.
Ultimately however, the date of Herod's death needs to be determined. According to conventional dating following Josephus, he died between a lunar eclipse (March 13, 4 BC) and the Passover (April 12). There remains little doubt that Herod probably did die in 4BC, as opposed to any other date. Josephus himself dates it to 4BC on numerous occasions, and Herod's sons dated their reigns from 4BC. However, it has been long recognized (e.g. William Whiston in his translation) that the time given is too short for the events mentioned prior to Herod's death and after the eclipse. It is therefore probably best to put Herod's death in late 4BC. This may explain why Appian places Herod's accession de jure in 39BC instead of 40BC as does Josephus (since Herod ruled 36 years, 37 in Josephus as per Jewish counting, and the possible closeness to 3BC, may have made Appian count 36 years back from 3BC to get 39BC). Moreover, the dating of Herod's sons seems to necessitate that Herod died no earlier than September, 4 BC.[Ken Doig, New Testament Chronology]
The 15th Year of Tiberius
The chronology of Jesus in Luke 3:23 makes him to be "about 30 years" in the 15th year of Tiberius. There are two things to discuss here. The first year of Tiberius would have certainly been reckoned from 14 AD when he became emperor, disregarding any coregencies or de jure claims for an earlier date ("co-princeps" with Augustus in 12 AD). [Suetonius, Tiberius 21] This is how his reign would have been commonly dated by anyone as Josephus does [Antiquities 18.4.6; 18.6.10], regardless of locale or regional calendars, even though they continued to use the month names of the land (e.g. in Egypt, Egyptian months: "4th year of Hadrian, Pharmouthi 27" [Loeb Classical Library (L266), C. C. Edgar, Select Papyri Vol. 1 (1970), p.207]). This would be especially true decades after when only the universal Roman reckoning would've remained recognizable for a Roman emperor (Josephus uses Jewish reckoning for Judean kings and Roman for Roman emperors).
It's true that some manuscripts have Josephus date the death of Philip to Tiberius' 22nd year instead of 20th in Antiquities 18.4.6, which, if authentic, could be interpreted either that the event took place two years later (35/36 instead of 33/34), or that Tiberius' reign was reckoned from 12 AD instead of 14. But later, in 18.6.10, Josephus clearly assigns a beginning date for Tiberius' reign by saying he reigned 22 years, 5 months, 3 days (a few weeks short), which means, since he died in early 37 AD, he must've counted his first year from 14 AD. Suetonius does the same. [Tiberius 73]
"About" 30 Years of Age
The statement, "about 30" is, like the entire verse, in accordance with how the Greek writers described someone. [e.g. in contracts, Loeb Classical Library ibid] It could be argued that Luke intended Jesus' age to be exactly 30, despite the relative description, "about", because documents used the same phrase to describe specific numbers (e.g. "Apollonius, aged about 26 years, of medium height, with fair complexion, long face, and a scar on the right foot..." [Loeb Classical Library, ibid., p.37]; "Psennesis also called Krouris son of Horus, Persian of the Epigone, of the village of Gotnit in the lower toparchy of the Latopolite nome, aged about 45 years, of medium height or under, dark-skinned, rather curly-haired, long-faced, straight-nosed, with a scar on the under lip..." [p.83]).
But these documents were written in present times by personal family members or close acquaintances. And they still approximated, unlike other timekeeping (e.g. "for 16th months...in Caesar's 17th year" [p.47]; a boat leased for 60 years [p.113], or a papyrus marsh for 3 years [p.125]). This is because of the minor amount of uncertainty that can exist about the age of an individual that doesn't need to relate in a legally-binding document.
Sometimes the person's age was unknown ("aged over 30" [p.251]), sometimes for brevity's sake ("[they] who are minors" [pp.103, 105]; cf the adoption of a son who is "about 2 years old" [p.31]), and sometimes the person stated the age without the "about" [pp.165, 249] which shows these are indeed approximations regardless of their almost universally specific nature. These were exceptions, so it's clear that despite the specific numbers found in the vast majority of private contracts, these were still approximations and Luke used it as such. So we don't need to suppose Jesus must've been exactly 30 years old in Tiberius' 15th year.
In conclusion, I think late 6 to mid 5 BC is the best estimate for the year of Christ's birth. No other year is within the boundary of both the historical and biblical evidence.
The Star of Bethlehem
Closely tied with the year of Jesus' birth is the star of Bethlehem which the Magi saw. There have been many suggestions for what astronomical event it could have been, for many dates close to 4 BC, if we take it as a natural event. Naturally, the correct way to start the question is to determine whether the star was natural or supernatural. To do this, let's start with the question of who the Magi were.
We can be certain that they were not high-ranking in Judaism religious Jews. This is illustrated by the fact that they went to Jerusalem to inquire where the King was, whereas Herod had to call the chief priests to tell him it was in Bethlehem (Matthew 2:5-6). This shows that they were not trained in the intimate details of the tradition of the Jewish religion. However, they seem to have known the important particulars of the Messianic expectations of the Jews, seeing how they designate the Messiah by the title "king of the Jews", reminiscent of the common Jewish expectation of a Jewish king (John 6:14-15), unlike the ideas about the Messiah coming from inspiration which see him exclusively as the salvation and redeemer of Israel and the world (Luke 1:46-56, 2:28-32, etc; only in Luke 1:52 do we see a hint of the Messiah as ruler, and this is barely anything like King of the Jews, because it compares God's glory over rulers). In fact, they most likely got this from Jews in Babylon, and there probably were enough such to have an influence, seeing from Josephus' account of the Babylonian Jew, Zamaris, and his 500 Jewish cavalry who came around 20 BC and became king of east of Judea.
But it is unlikely that the Magi were Jews at all, or converts from a long time, for two factors. First, and very speculatively, they do not seem to have known the way to Bethlehem as per Herod directing them to it (Matthew 2:8). Not much can be made from the phrase, "and he sent them", Herod sending the Magi toward Bethlehem as if they didn't know the way. Indeed, their wealth, seen by their overly generous gifts (Matthew 2:11) seems to strengthen the supposition that they used to be trained Zoroastrian priests from Babylon. Second, they apparently did not know Herod's murderous and jealous nature to go announce to him their search for a "king of the Jews", whereas it makes total sense for strangers to come to the capital of a different country for, what would seem to them, straightforward information. We cannot suppose that foreigners, from Babylon, would have known Herod's character from the Jews in Babylon; it is an unwarranted assumption. In any case, it would be impossible for Jews from Israel not to have known. They were most likely recent converts to Judaism, seeing they probably had not been circumcised or were not practicing the other tenets of Judaism at the time, and were perhaps converted to Judaism by the revelation they had, which would have been not too long before the appearance of the Star. Had they been born Jews, they would have observed the Feasts such as Passover, and would have known about Herod. That Matthew 2:12 seems to imply they found an alternate route out of Israel by themselves, does not need to maintain they were well acquainted with the land; they simply could have went a different way, many of which would have probably been available. Finally, Herod turns to the chief priests for an answer as to where the Messiah is to come from. The Magi would have been familiar with this tradition if they were such devout Jews willing to come all this way, having been chosen by God for a revelation.
The answer is probably that they had a revelation about the Star of Bethlehem's appearance and guidance to the Messiah, whom they knew to be the King of the Jews through the common Jewish beliefs they probably would have known about (being priests). The revelation may have been direct enough, or expedient enough, to not allow them or give them time to become Jews by being circumcised, and so on. In fact, it would explain why they still practiced the ways of the Magi; they were simply applying their knowledge of the stars to see which star God was using to lead them to the Messiah. It would also explain why God would use a Star of Bethlehem to prove to them the King's birth instead of just telling them about it in a dream; it may have been proof enough for them which converted them to the Jewish religion, after which they simply took the dreams' words at faith without the need of proof as in the warning about not to return to Herod.
The above does not affect the determination of the Star of Bethlehem as natural or supernatural; as extraordinary looking from the naked eye, or if it appeared as a normal star. It only serves to support the following conclusion that the star was a normal looking star and there was nothing special in its physical appearance.
We can see that Herod and no one else knew when the star had appeared because he would not have inquired from the Magi "secretly" the time of the appearance of the Star (Matthew 2:7) and it would have been information which would have been known throughout all of the Levant, if it was seen from Babylon a few days to weeks ago. Even if one supposes that the Star of Bethlehem had signified conception and therefore came about 9 months to one year prior to the Magi's arrival, it would still have been remembered and Herod would not have needed to inquire secretly from the Magi. The star was therefore nothing more special than perhaps a very bright star so the Magi would have been able to identify the sign of the King's birth.
The second appearance of the Star of Bethlehem was similarly indistinguishable and "incognito" from the other stars as the first (Matthew 2:9). Since the star was no special astronomical phenomenon, we can exclude comets, supernovae, and special star conjunctions that would have been very distinguishable. The list therefore excludes Haley's comet in 12 BC, the comet of 10 BC, and the supernovae of 5 and 4 BC. The Star of Bethlehem was either hovering over Israel or not continuously there, or else the Magi would have never stopped at Jerusalem but would have continued following it to Bethlehem. If it is suggested that the star simply stood over Israel, one can wonder why Herod would not have come with the Magi himself, following the star, but instead sends them to Bethlehem to find Christ, which would have involved searching many households (for there to be victims for Herod's slaughter). It is possible the star didn't start going toward Bethlehem until after their arrival at Jerusalem. However, as the Magi would have undoubtedly pointed the Star out to Herod, when it started going toward Bethlehem, Herod would have most likely followed it as well, and would have reached Christ not too long after the Magi. Furthermore, Matthew 2:10 implies the Star had appeared anew. Therefore, the most logical conclusion is that the Star of Bethlehem disappeared before they had reached Jerusalem, likely after the first night, the night of their departure, but no definite time prior to their arrival in Jerusalem can be given. Since their journey would have taken a week or two at most, this is further evidence for the exclusion of astronomical phenomena like the supernova of 5 BC, which lasted 70 days.
With the above, we should turn to other known astronomical events. The triple conjunction of Saturn and Jupiter in 6 BC does not qualify because its motion did not disappear and reappear. There are certainly tantalizing astronomical phenomena, but the clearest evidence that this was no natural phenomena comes from the fact that the Star guided the Magi to the exact house in Bethlehem, something no natural celestial body is likely to have done, even on accident (speaking in relative terms of course, since its path would have been of course predetermined by God). The best conclusion is therefore that the Star of Bethlehem was a regular-looking but supernatural in origin star which had a marker as the king's star for the Magi somehow, maybe by being bright, and is nothing that can be verified from what we know, and thus its date cannot be known through this method.
The month of Jesus' birth
If we are completely unsure of the year of Jesus' birth, how much more of the month, or the date. Nevertheless, as before, there are subtle factors that give relative periods. The timeless objection that the shepherd's could not have had flocks out on the field during winter (Luke 2:8) is probably true. The verse doesn't seem to indicate anything but that their flocks were outside in the field, and a winter night would be very improbable for this.
Chronology of the infancy of Christ
That the star had appeared shortly before the Magi's appearance to Herod is evident by the fact that they must have started their journey as soon as they had seen it rise (otherwise why wait long if the king had been born?). The star they saw while in Babylon was certainly to indicate that the King was born, not conceived, as strongly implied by Matthew 2:1-2. Although Matthew 2:2 can be said to allow the Magi to have seen the star at Jesus' conception, it implies that it was at his birth, and does not make much sense to have the announcement of the king's conception as opposed to his birth. Also, Matthew 2:8-9 implies the king had been born by then, otherwise the Magi would have had impeccably lucky timing at arriving exactly on the day of the Messiah's birth. If it is posited that they arrived some time after the birth of Christ, like a few months, it would make no sense for the Magi to see a star announcing the Messiah's conception, and then wait more than the 9 months of the king's birth. On the other hand, it is also equally unlikely that they saw the star announcing the King's conception and came within a few days of his birth, for the simple fact that the Magi presuppose the King had already been born (Matthew 2:2). One can answer that perhaps Matthew's Jewish phrasing changed Matthew 2:2 to say the King who was born, and the Magi did not expect such with certainty. We can't know with certainty whether the first sighting of the Star of Bethlehem was at conception or birth, despite the strong implication of the text of the latter. However, those who suppose that the first appearance of the Star signified conception have to presuppose a second time the holy family lived in Bethlehem, returning to it a year or so later. Such a return has no grounds. The only reason they went to Bethlehem in the first place was because of the census, and there is no logical reason to return to Bethlehem when they had nothing there (Luke 2:7) and no reason to be there.
The year of the start of Christ's ministry
There are three chronological markers that are used for the determination of the year of the start of Christ's ministry. The first is the mention of the Temple having been under construction for 46 years in John 2:20. We know from Josephus that this was begun some time after the Passover of 20 BC, and certainly the project could not have taken off until late 20 BC, and archaeological evidence suggests it started in 19 BC, due to determination that the stone quarry from where stones were taken did not start until 19 BC, based on coin evidence. From late 20BC/early 19BC, 46 years brings us to early 28 AD, and the text in John suggests this was the first Passover of Jesus' ministry.
Second, the start of John the Baptist's ministry is given as the 15th year of Tiberius (Luke 3:1). The text (Luke 3:1-18,21) does not necessitate a period of more than a month or two, and in fact seems to imply that Jesus was amongst the first to be baptized by John the Baptist.
Third, Luke 3:23 states that Jesus was "about thirty years" when he started his ministry; this suggests he was either 29 years and some months, or 30 years and some months, perhaps due to Luke's lack of knowledge of the month of Christ's birth, but only the year. If all Luke knew was late 4BC (the 21st year of Augustus) as the birth-year of Jesus, and the 15th year of Tiberius (28/29AD; Luke 3:1) as the starting year of his ministry, then going from Augustine's 21st to Tiberius' 1st year we have 17 years in late 14 AD. From late 14 until late 27 we have 13 years, making the total 30, and another few months to half year to reach early 28 would make Jesus "about 30 years old" instead of something more technical like "30 years old" or "30 and a half" years old. Tiberius' 15th year may have been reckoned as 28 AD, especially if all of 4 BC was reckoned as Augustine's 21st.
The length of the ministry of Jesus
The year of the crucifixion
- The way the Magi would have seen a "regular-looking" star as the King's star is possibly through a dream forecasting the star's appearance where there should be no star, and the righteous Magi, knowing the stars' positions, would have known there should be no star there, whereas the Zoroastrian priests would have simply been confused by it.
- Res Gestae Divi Augusti, 4 - dates 14AD as Augustus' 37th year. Back-dating from 14 AD to 1 BC (14 years) and from 1 BC to 4 BC we get 4 BC as the 20th year. Since Jesus would have been born in late 4 BC, it would have been Augustine's 21st regnal year. If Luke knew Jesus' birth year as Augustine's 21st, he would have known the nativity was no earlier than late 4BC and no later than mid 3BC, and so he would not have considered Jesus to be about 31 years old, since mathematically speaking the number of years from January 1, 4BC to January 1, 28AD are 31, which one might think would be a better estimate if Luke considered the nativity in 4BC and the start of the ministry in 28AD as the chronology demands, but this resolves the issue, and Luke is correct in saying Jesus was about 30 years old, having been born in 4BC and starting his ministry in 28 AD.