They established the time of the earthquake between 26 and 36 CE. As far as I know there is only record of this quake in the Bible. They also used data from the Jewish calendar, astronomical calculations, as well as the different Biblical accounts to decide on a date.
They didn't decide on a date for the quake. They discussed how the crucifixion is commonly dated to April 3, AD 33 and they discussed how the quake was dated to AD 31 ± 5 years. Then they discussed whether the quake dated to AD 31 ± 5 years could be identified with the quake mentioned in the gospel of Matthew. They consider three different possibilities and leave the choice between them as inconclusive. They do not settle on April 3, AD 33 as the date of the quake. If the MSNBC report gave the impression they did, then again one should always consult the original journal article and not a summary of it in the media.
There is also no record of a mass resurrection during that time, so the corrupt text could have been caused by a transcription error. An earthquake could have caused bodies "to stand up." Afterwards people would have wondered amongst the tombs.
What specific transcription error are you thinking of in the text that we have? The text quite deliberately refers to a resurrection, and that is both historically problematic (a sign that the quake is more in the realm of legend or allegory) as you note as well as theologically problematic (it would deny Jesus the privilege of being the "firstborn" and "firstfruits" of the resurrection, as claimed by Paul). An event in which the quake merely causes the bodies to "stand up" is reminiscent of Johannes Greber's dubious translation: "The earth quaked, and the rocks were shattered. Tombs were laid open, and many bodies of those buried there were tossed upright. In this posture they projected from the graves and were seen by many who passed by the place on their way back to the city" (The New Testament -- A New Translation, 1937). The NWT follows Greber's lead and also imposes parantheses to artificially mark part of the text as a digression:
"The earth quaked, and the rock-masses were split. And the memorial tombs were opened and many bodies of the holy ones that had fallen asleep were raised up, (and persons, coming out from among the memorial tombs after his being raised up, entered into the holy city,) and they became visible to many people."
In both readings the bodies in the graves are not resurrected but merely "raised up" or "tossed upright" and remained in their tombs, while other persons walking among the tombs went into the city (not those tossed upright in the tombs). This is a very contrived interpretation that can be criticized on a number of grounds. First we read that "tombs were opened" (mnèmeia aneòkhthèsan), and this is an allusion to the Vision of Dry Bones (a primary OT source on the resurrection belief) where God says " I will open your tombs (anoig ò hum ò n ta mn è mata) and cause you to come up (anax ò ) out of your tombs (ek t ò n mn è mat ò n), my people; and I will bring you into (eisax ò hu mas) the land of Israel" (Ezekiel 37:12 LXX); both anoig ò and aneòkhthèsan are forms of the verb anoigein "to open up" (first person singular active present and third person plural passive aorist, respectively). The two words occur elsewhere in descriptions of the resurrection: "And after these things a trumpet blast, and the tombs will be opened (mnèmeia anoikhthesontai) and the dead will rise up uncorrupted" (Greek Apocalypse of Ezra 4:36). Second, the Greber translation and the NWT refer only the bodies being "raised up" or "tossed upright", and Greber further adds a reference to "this posture". But this ignores what is stated in text itself. A nominal form (egersin) of the same word (egerein "to rise up") is used in the next verse (v. 53) to refer to Jesus' resurrection. It thus should be understood as resurrection in the preceding verse, particularly since there the holy ones are said to be kekoimèmenòn "sleeping" (the state of death from which the dead are awakened and raised up). Compare 1 Thessalonians 4:14-16 where the "sleeping" dead are awakened by the trumpet call and rise from the dead. Second, v. 53 attributes to following actions to the holy ones: they "came out" (exelthontes, masculine plural in agreement with "holy ones" in v. 52) of the tombs, they "came into" (eisèlthon) the city, and then they "appeared" (enephainisthèsan) to many. These actions presume an agency not attributable to inanimate bodies, not even by assuming that others picked them up and carried them into the city. This motivates the NWT's parenthetical insertion of "persons" into the text to make the reference to the first two actions attributable to conscious agents, even tho there is no basis for "persons" in the text. Another contrivance is the statement that these persons were "coming out from among the memorial tombs" in the NWT, which implies they were on the surface walking between the tombs, whereas they clearly are exiting the tombs themselves (exelthontes ex tòn mnèmeiòn "came out from the tombs"). And this language is clearly borrowed from Ezekiel 37:12 LXX: After opening the tombs, Yahweh would " cause you to come up (anax ò ) out of your tombs (ek t ò n mn è mat ò n)" , and then he would "bring you into" (eisaxò) the land of Israel. This is parallel to the opening of the tombs being followed by the dead "coming out" (exerkhomai) "out of the tombs" (ex tòn mnèmeiòn) and then "coming into" (eiserkhomai) the holy city. So the passage in its construction is clearly a resurrection narrative, and the Greber and NWT renderings attempt to mitigate this.
That's the text as it stands. But it is also a difficult text, which is why many scholars believe that it has been redacted. Even the Society suggests that this is a possibility (Watchtower, 1/1/1961, p. 30). The passage contains unusual language used elsewhere by Paul in reference to the resurrection, all of which is theologically loaded: "bodies" (used by Paul in 1 Corinthians 15:35-44), "saints" (widely used by Paul to refer to the faithful), and "who had fallen asleep" (used in reference to those who will be resurrected in 1 Corinthians 15:20 and 1 Thessalonians 4:14). The noun egersis is also nowhere else applied to Jesus' resurrection in the NT (as opposed to the usual term anastasis), although the verbal form is commonly used. The motivation for the revision can be found in Paul: "But Christ has in fact been raised from the dead, the first-fruits of all who have fallen asleep...All men will be brought to life in Christ but all of them in their proper order: Christ as the first-fruits and then after the coming of Christ, those who belong to him" (1 Corinthians 15:20-23). This would explain why the text was glossed: Christ must first rise and leave his tomb prior to the saints, who only do so "after his resurrection". This however is a contrivance: they are still raised at the moment the temple veil is torn, they inexplicably wait in their graves until Easter Sunday, and the Matthean account has the centurion exclaim "This was the son of God" in amazement in the very next verse. The centurion is not only commenting on the earthquake, but "the earthquake and the things that happened". The likely original account thus would have contradicted Paul and have the resurrection of the saints occur at that very moment, climaxing the events leading to the confession by the centurion.
There is also possibly textual evidence that the text in Matthew may have been altered. One of its earliest witnesses was the Diatesseron, a gospel harmony produced by Tatian towards the end of the second century. This harmony was in turn based on the one produced by Justin Martyr several decades earlier. The Pepysian Harmony and the Ephrem Commentary both attest the Diatessaron reading as follows:
And with that, the veil that hung in the temple before the high altar burst in two pieces, the earth quaked, and the stones burst, and the dead men arose out of their graves. And entering the holy city, they appeared to many. And the centurion and those with him, who stood facing Jesus, saw the earthquake and what took place, and said with awe, "Truly this was the Son of God!"
Here the gloss does not appear and the appearance of the risen dead in Jerusalem occurs at the same time as Jesus' death and was witnessed by the centurion. This reading makes better sense with the context. It also omits the greater detail of the canonical account in this passage, lacking the Pauline-like language described earlier.
There is also an open question of whether the tradition underlying the passage is relevant to the claim by Hymenaeus and Philetus that "the resurrection has already taken place" (2 Timothy 2:16-17), as the present passage on its face would seem to make a similar claim. Others see a connection with traditions of the harrowing of hell during Jesus' death and resurrection. The Gospel of Nicodemus (fifth or sixth century AD) builds on the Pericope Zombiae in ch. 17:
It is more marvelous that he rose not alone from the dead, but did raise up alive many other dead out of their sepulchres, and they have been seen of many in Jerusalem. And now hearken unto me; for we all know the blessed Simeon, the high priest which received the child Jesus in his hands in the temple. And this Simeon had two sons, brothers in blood and we all were at their falling asleep and at their burial. Go therefore and look upon their sepulchres: for they are open, because they have risen, and behold they are in the city of Arimathaea dwelling together in prayer. And indeed men hear them crying out, yet they speak with no man, but are silent as dead men. But come, let us go unto them and with all honour and gentleness bring them unto us, and if we adjure them, perchance they will tell us concerning the mystery of their rising again.
