These are the traditional dates which most scholars in this day accord the four canonical gospels. There is a growing movement to give earlier dates, typified by John A.T. Robinson's Dating the Gospels. In the 19th and early 20th century scholars tended to give late dates. May skeptics on the net go with the 19th century tendency, and ascribe John to mid or late second century, but they are quite out of step with scholarship and the times.
The major problem for dating of all four Gospels is what to do with the fall of temple? Does one assume that references to it existed before the fall, as prophecy and thus Jesus was truely a prophet, or does one make the naturalist assumption and assume that these things were invent after the fall of the temple, thus dating all gospels after 70? I will sidestep proofs and take the naturalist assumption. I do this, not because I believe Jesus didn't prophesy it, but for rhetorical reasons, because the skeptics are not going to assume otherwise. IF we are to do valid apologetics, we must show them that the consequences of their assumptions are not necessarily as skeptically supportive as they think.
Gap Between Events and Gospels
One way we can Do this is to disabuse them of one of their most egregious assumptions which is based upon this dating method. Even for those who accept these mainstream dates, they still make a classic mistake in assumption. One of the general charges voiced by skeptics of all stripes is the idea that a long gap exists between the events portrayed in the Gospels,and the writing of the documents themselves. Jesus mythers in particular, those who believe that Jesus did not exist in history as a flesh and blood man, urge that this gap invalidates any evidence for Jesus that comes from a latter period, although why that would be they can't say.
This notion of a gap where no "evidence" exists for the existence of Jesus is really pure bunk. While is not one finished document we can point to, we know certainly that the canonical gospels, or the material that became the canonicals was in circulation as early as the 40s and 50s. We know that written documents existed as early as the 40s. The canonicals were the prodcut of a process and such they went thorugh man redactions.Those redactions cover a large span of time.
The actual fact, which is proven with the scientific methods of textual criticism, is that traces of the same material which can be seen in the canonical gospels can be found as far back as AD50, in written form, suggesting that it has a much older circulation on oral form. I will argue that this is the case and that what we have as the canonical Gospels of today are the finished products after a long process of editing in which the same basic material forged by eye witness testimony is still preserved and passed on.
Traces of Gospel Material in Gap
The circulation of Gospel material can be showen in four areas:
(1) Oral tradition
(2) saying source Material
(3) Non canonical Gospels
(4) traces of pre Markan redaction(PMR) (canonical material that pre-date Mark, assumed the to be the first Gosple)
Oral tradtion in two major sources
(1) Pauline references to sayings
The great scholar Edgar Goodspeed held that oral tradition was not haphazard rumor but tightly controlled process,and that all new converts were required to learn certain oral traditions and spit them back from memory:
This web page is placed in the public domain by Peter Kirby and Wally Williams
Our earliest Christian literature, the letters of Paul, gives us glimpses of the form in which the story of Jesus and his teaching first circulated. That form was evidently an oral tradition, not fluid but fixed, and evidently learned by all Christians when they entered the church. This is why Paul can say, "I myself received from the Lord the account that I passed on to you," I Cor. 11:23. The words "received, passed on"  reflect the practice of tradition—the handing-down from one to another of a fixed form of words. How congenial this would be to the Jewish mind a moment's reflection on the Tradition of the Elders will show. The Jews at this very time possessed in Hebrew, unwritten, the scribal interpretation of the Law and in Aramaic a Targum or translation of most or all of their Scriptures. It was a point of pride with them not to commit these to writing but to preserve them.
1 Corinthians 15:3-8 has long been understood as a formula saying like a creedal statment.
For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
1Cr 15:4 And that he was buried, and that he rose again the third day according to the scriptures:
1Cr 15:5 And that he was seen of Cephas, then of the twelve:
1Cr 15:6 After that, he was seen of above five hundred brethren at once; of whom the greater part remain unto this present, but some are fallen asleep.
1Cr 15:7 After that, he was seen of James; then of all the apostles.
1Cr 15:8 And last of all he was seen of me also, as of one born out of due time.
Two problems: (1) Doesn't conform to a canonical reading; (2) seems to contradiction the order of appearances of the epiphanies (in fact doesn't even mention the women). Nevertheless it is in general agreement with the resurrection story, and seems to indicate an oral tradition already in circulation by the AD 50s, and probably some time before that since it has had tome to be formed into a formulamatic statement.
The problem with showing Oral tradition is that we have to find it in writing and this is essentually impossible. But we do find references to it in Paul. We find the doxology and other sayings that to which he alludes.
(2)The nature of pericopes
The nature of the pericopes themselves shows us that the synoptic gosples are made up of units of oral tradition. Many skpetics seem to think that Mark indented the story in the Gospel and that's the first time they came to exist. But no, Mark wrote down stories that the chruch had told for decades. Each unit or story is called a "pericope" (per-ic-o-pee).
pronounced "pur-IH-cuh-pee") - an individual "passage" within the Gospels, with a distinct beginning and ending, so that it forms an independent literary "unit"; similar pericopes are often found in different places and different orders in the Gospels; pericopes can include various genres (parables, miracle stories, evangelists' summaries, etc.)
On this basis Baultmann developed "form cricisim" because the important aspect was the form the oral tradition too, weather parable, narration, or other oral form.
Saying Source Material
(1)Gospel of Thomas
The Gospel of Thomas which was found in a Coptic version at Nag Hammadi, but also exists in another form in several Greek fragments, is a prime example of a saying source. The narratival elements are very minimul, amounting to things like "Jesus said" or "Mary asked him about this,and he said..." The Gospel is apt to be dismissed by conservatives and Evangelicals due to its Gnostic elements and lack of canonicity. While it is true that Thomas contains heavily Gnostic elements of the second century or latter, it also contains a core of sayings which are so close to Q sayings from the synoptics that some have proposed that it may be Q (see Helmutt Koester, Ancient Christian Gospels). Be that as it may, there is good evidence that the material in Thomas comes from an independent tradition,t hat it is not merely copied out of the synoptics but represents a PMR.
KIrby quoting:Ron Cameron comments on the attestation to Thomas (op. cit., p. 535):
"The one incontrovertible testimonium to Gos. Thom. is found in Hippolytus of Rome (Haer. 5.7.20). Writing between the years 222-235 C.E., Hippolytus quoes a variant of saying 4 expressly stated to be taken from a text entitled Gos. Thom. Possible references to this gospel by its title alone abound in early Christianity (e.g. Eus. Hist. Eccl. 3.25.6). But such indirect attestations must be treated with care, since they might refer to the Infancy Gospel of Thomas. Parallels to certain sayings in Gos. Thom. are also abundant; some are found, according to Clement of Alexandria, in the Gospel of the Hebrews and the Gospel of the Egyptians. However, a direct dependence of Gos. Thom. upon another noncanonical gospel is problematic and extremely unlikely. The relationship of Gos. Thom. to the Diatessaron of Tatian is even more vexed, exacerbated by untold difficulties in reconstructing the textual basis of Tatian's tradition, and has not yet been resolved."
(Ron Cameron, ed., The Other Gospels: Non-Canonical Gospel Texts (Philadelphia, PA: The Westminster Press 1982), pp. 23-37.)
Kirby--In Statistical Correlation Analysis of Thomas and the Synoptics, Stevan Davies argues that the Gospel of Thomas is independent of the canonical gospels on account of differences in order of the sayings.
In his book, Stephen J. Patterson compares the wording of each saying in Thomas to its synoptic counterpart with the conclusion that Thomas represents an autonomous stream of tradition (The Gospel of Thomas and Jesus, p. 18):
(Kirby quoting Patterson)"If Thomas were dependent upon the synoptic gospels, it would be possible to detect in the case of every Thomas-synoptic parallel the same tradition-historical development behind both the Thomas version of the saying and one or more of the synoptic versions. That is, Thomas' author/editor, in taking up the synoptic version, would have inherited all of the accumulated tradition-historical baggage owned by the synoptic text, and then added to it his or her own redactional twist. In the following texts this is not the case. Rather than reflecting the same tradition-historical development that stands behind their synoptic counterparts, these Thomas sayings seem to be the product of a tradition-history which, though exhibiting the same tendencies operative within the synoptic tradition, is in its own specific details quite unique. This means, of course, that these sayings are not dependent upon their synoptic counterparts, but rather derive from a parallel and separate tradition."
Stevan L. Davies, The Gospel of Thomas: Annotated and Explained (Skylight Paths Pub 2002)
(2) Pauline references
Koster theorizes that Paul probably had a saying source like that of Q avaible to him. Paul's use of Jesus' teachings indicates that he probably worked from his own saying source which contained at least aspects of Q. That indicates wide connection with the Jerusalem chruch and the proto "Orthodox" faith.