Mark 13.19-20: 19 "For those days will be a time of tribulation such as has not occurred since the beginning of the creation which God created, until now, and never should. 20 And if the Lord had not shortened those days, all flesh would not have been saved; but for the sake of the elect, whom He chose, He shortened the days."
Matthew 24.21-22: 21 For then there will be a great tribulation, such as has not occurred since the beginning of the world until now, nor ever should. 22 Unless those days had been cut short, all flesh would not have been saved; but for the sake of the elect those days will be cut short.
Daniel 12.1-3: 1 "Now at that time Michael, the great prince who stands guard over the sons of your people, will arise. And there will be a time of distress such as never occurred since there was a nation until that time; and at that time your people, everyone who is found written in the book, will be rescued. 2 And many of those who sleep in the dust of the ground will awake, these to everlasting life, but the others to disgrace and everlasting contempt. 3 And those who have insight will shine brightly like the brightness of the expanse of heaven, and those who lead the many to righteousness, like the stars forever and ever."
Matthew 24.21-22: 21 For then there will be a great tribulation, such as has not occurred since the beginning of the world until now, nor ever should. 22 Unless those days had been cut short, all flesh would not have been saved; but for the sake of the elect those days will be cut short.
Daniel 12.1-3: 1 "Now at that time Michael, the great prince who stands guard over the sons of your people, will arise. And there will be a time of distress such as never occurred since there was a nation until that time; and at that time your people, everyone who is found written in the book, will be rescued. 2 And many of those who sleep in the dust of the ground will awake, these to everlasting life, but the others to disgrace and everlasting contempt. 3 And those who have insight will shine brightly like the brightness of the expanse of heaven, and those who lead the many to righteousness, like the stars forever and ever."
I have elsewhere noted that Mark 13.20 is in the past tense: the sense is that those days of tribulation were shortened, which is why not all flesh did perish.
Here and now, however, I wish to point out both (A) that the motif of shortening the days is pretty common in ancient Jewish literature and (B) that this particular instance of shortening the days differs from the other instances, which would include at the very least:
1 Enoch 80.2-4: 2 And in the days of the sinners the years shall be shortened, and their seed shall be tardy on their lands and fields, and all things on the earth shall alter and shall not appear in their time; and the rain shall be kept back and the heaven shall withhold it. 3 And in those times the fruits of the earth shall be backward, and shall not grow in their time, and the fruits of the trees shall be withheld in their time. 4 And the moon shall alter her order, and not appear at her time.
Wisdom of Sirach 36.8: 8 Hasten the day, and remember the appointed time, and let people recount thy mighty deeds.
4Q385, fragment 3, lines 1-7: 1 [...] under my grief. ~ [... and my heart] 2 [dis]turbs my soul. And the days will pass rapidly until [all the sons of] 3 man say, «Are not the days hastening on so that the children of Israel can inherit [their land?»] 4 And YHWH said to me: «I will not sh[u]n your face, Ezekiel; s[ee,] I measure [time and shorten] 5 the days and the years [...] 6 a little, as you said to [...] 7 [For the mou]th of YHWH has said these things.» [....]
1 Corinthians 7.29-31: 29 But this I say, brethren, the season has been shortened [ὁ καιρὸς συνεσταλμένος ἐστίν], so that from now on those who have wives should be as though they had none; 30 and those who weep, as though they did not weep; and those who rejoice, as though they did not rejoice; and those who buy, as though they did not possess; 31 and those who use the world, as though they did not make full use of it; for the scheme of this world is passing away [παράγει γὰρ τὸ σχῆμα τοῦ κόσμου τούτου].
Barnabas 4.3: 3 Τὸ τέλειον σκάνδαλον ἤγγικεν περὶ οὗ γέγραπται, ὡς Ἐνὼχ λέγει, «Εἰς τοῦτο γὰρ ὁ δεσπότης συντέτμηκεν τοὺς καιροὺς καὶ τὰς ἡμέρας, ἵνα ταχύνῃ ὁ ἠγαπημένος αὐτοῦ καὶ ἐπὶ τὴν κληρονομίαν ἥξει.» / 3 The final stumbling block is at hand, about which it has been written, just as Enoch says, "For this reason the Master has cut short the seasons and the days, that his beloved may hurry and arrive at his inheritance."
2 Baruch 20.1-2: 1 "Therefore behold! the days come, and the times shall hasten more than the former, and the seasons shall speed on more than those that are past, and the years shall pass more quickly than the present (years). 2 Therefore have I now taken away Zion, that I may the more speedily visit the world in its season."
2 Baruch 54.1: 1 And I besought the Mighty One, and said: "You alone, O Lord, know of aforetime the deep things of the world, and the things which befall in their times You bring about by Your word, and against the works of the inhabitants of the earth you do hasten the beginnings of the times, and the end of the seasons you alone know."
2 Baruch 83.1: 1 For the Most High will assuredly hasten His times, and He will assuredly bring on His hours.
4 Ezra 2.13: 13 Ask and you will receive; pray that your days may be few, that they may be shortened. The kingdom is already prepared for you; watch!
Talmud, Baba Metzia 85b: Elijah used to frequent Rabbi's academy. One day — it was New Moon — he was waiting for him, but he failed to come. Said he to him [the next day]: "Why didst thou delay?" He replied: "[I had to wait] until I awoke Abraham, washed his hands, and he prayed and I put him to rest again; likewise to Isaac and Jacob." "But why not awake them together?" "I feared that they would wax strong in prayer and bring the Messiah before his time."
Wisdom of Sirach 36.8: 8 Hasten the day, and remember the appointed time, and let people recount thy mighty deeds.
4Q385, fragment 3, lines 1-7: 1 [...] under my grief. ~ [... and my heart] 2 [dis]turbs my soul. And the days will pass rapidly until [all the sons of] 3 man say, «Are not the days hastening on so that the children of Israel can inherit [their land?»] 4 And YHWH said to me: «I will not sh[u]n your face, Ezekiel; s[ee,] I measure [time and shorten] 5 the days and the years [...] 6 a little, as you said to [...] 7 [For the mou]th of YHWH has said these things.» [....]
1 Corinthians 7.29-31: 29 But this I say, brethren, the season has been shortened [ὁ καιρὸς συνεσταλμένος ἐστίν], so that from now on those who have wives should be as though they had none; 30 and those who weep, as though they did not weep; and those who rejoice, as though they did not rejoice; and those who buy, as though they did not possess; 31 and those who use the world, as though they did not make full use of it; for the scheme of this world is passing away [παράγει γὰρ τὸ σχῆμα τοῦ κόσμου τούτου].
Barnabas 4.3: 3 Τὸ τέλειον σκάνδαλον ἤγγικεν περὶ οὗ γέγραπται, ὡς Ἐνὼχ λέγει, «Εἰς τοῦτο γὰρ ὁ δεσπότης συντέτμηκεν τοὺς καιροὺς καὶ τὰς ἡμέρας, ἵνα ταχύνῃ ὁ ἠγαπημένος αὐτοῦ καὶ ἐπὶ τὴν κληρονομίαν ἥξει.» / 3 The final stumbling block is at hand, about which it has been written, just as Enoch says, "For this reason the Master has cut short the seasons and the days, that his beloved may hurry and arrive at his inheritance."
2 Baruch 20.1-2: 1 "Therefore behold! the days come, and the times shall hasten more than the former, and the seasons shall speed on more than those that are past, and the years shall pass more quickly than the present (years). 2 Therefore have I now taken away Zion, that I may the more speedily visit the world in its season."
2 Baruch 54.1: 1 And I besought the Mighty One, and said: "You alone, O Lord, know of aforetime the deep things of the world, and the things which befall in their times You bring about by Your word, and against the works of the inhabitants of the earth you do hasten the beginnings of the times, and the end of the seasons you alone know."
2 Baruch 83.1: 1 For the Most High will assuredly hasten His times, and He will assuredly bring on His hours.
4 Ezra 2.13: 13 Ask and you will receive; pray that your days may be few, that they may be shortened. The kingdom is already prepared for you; watch!
Talmud, Baba Metzia 85b: Elijah used to frequent Rabbi's academy. One day — it was New Moon — he was waiting for him, but he failed to come. Said he to him [the next day]: "Why didst thou delay?" He replied: "[I had to wait] until I awoke Abraham, washed his hands, and he prayed and I put him to rest again; likewise to Isaac and Jacob." "But why not awake them together?" "I feared that they would wax strong in prayer and bring the Messiah before his time."
What all these instances of shortening the days have in common is that the actual day in question, the day of the Lord or the end of all things, is apparently being made closer to the present time; the days are being cut short in the sense that there will be fewer of them between today and the last day.
This sense is entirely lacking in Matthew 24.21-22 = Mark 13.19-20. The days which are being cut short are not the days between today and the end but rather the days of the tribulation. This shortening may conceptually also hasten the end day, but it also may conceptually simply create a gap between the end of the tribulation and that day. That Mark 13.20 is in the past tense suggests the latter, so far as the gospel of Mark is concerned; the days of tribulation were cut short, creating an otherwise unexpected period of time after the tribulation but before the day of salvation, the coming of the Son of Man.
My hypothesis is that the author of Mark 13.20 thought of him/herself as living in that gap, looking back upon what he/she, like Josephus and Eusebius, regarded as the greatest such event of all time:
Josephus, Wars 0.1.1 §1-3: 1 Ἐπειδὴ τὸν Ἰουδαίων πρὸς Ῥωμαίους πόλεμον συστάντα μέγιστον οὐ μόνον τῶν καθ᾽ ἡμᾶς, σχεδὸν δὲ καὶ ὧν ἀκοῇ παρειλήφαμεν ἢ πόλεων πρὸς πόλεις ἢ ἐθνῶν ἔθνεσι συρραγέντων, οἱ μὲν οὐ παρατυχόντες τοῖς πράγμασιν, ἀλλ᾽ ἀκοῇ συλλέγοντες εἰκαῖα καὶ ἀσύμφωνα διηγήματα σοφιστικῶς ἀναγράφουσιν, 2 οἱ παραγενόμενοι δὲ ἢ κολακείᾳ τῇ πρὸς Ῥωμαίους ἢ μίσει τῷ πρὸς Ἰουδαίους καταψεύδονται τῶν πραγμάτων, περιέχει δὲ αὐτοῖς ὅπου μὲν κατηγορίαν ὅπου δὲ ἐγκώμιον τὰ συγγράμματα, τὸ δ᾽ ἀκριβὲς τῆς ἱστορίας οὐδαμοῦ, 3 προυθέμην ἐγὼ τοῖς κατὰ τὴν Ῥωμαίων ἡγεμονίαν Ἑλλάδι γλώσσῃ μεταβαλὼν ἃ τοῖς ἄνω βαρβάροις τῇ πατρίῳ συντάξας ἀνέπεμψα πρότερον ἀφηγήσασθαι Ἰώσηπος Ματθίου παῖς ἐξ Ἱεροσολύμων ἱερεύς, αὐτός τε Ῥωμαίους πολεμήσας τὰ πρῶτα καὶ τοῖς ὕστερον παρατυχὼν ἐξ ἀνάγκης. / 1 Whereas the war which the Jews made with the Romans has been the greatest of all those, not only that have been in our times, but, in a manner, of those that ever were heard of; both of those wherein cities have fought against cities, or nations against nations; while some men who were not concerned in the affairs themselves have gotten together vain and contradictory stories by hearsay, and have written them down after a sophistical manner; 2 and while those that were there present have given false accounts of things, and this either out of a humor of flattery to the Romans, or of hatred towards the Jews; and while their writings contain sometimes accusations, and sometimes encomiums, but no where the accurate truth of the facts; 3 I have proposed to myself, for the sake of such as live under the government of the Romans, to translate those books into the Greek tongue, which I formerly composed in the language of our country, and sent to the Upper Barbarians; 3 I am Josephus, the son of Matthias, by birth a Hebrew, a priest also, and one who at first fought against the Romans myself, and was forced to be present at what was done afterwards.
Eusebius, History of the Church 3.7.1-2: 1 It is fitting to add to these accounts the true prediction of our Savior in which he foretold these very events. 2 His words are as follows: "Woe unto them that are with child, and to them that give suck in those days! But pray that your flight be not in the winter, neither on the Sabbath day. For there shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be."
Eusebius, History of the Church 3.7.1-2: 1 It is fitting to add to these accounts the true prediction of our Savior in which he foretold these very events. 2 His words are as follows: "Woe unto them that are with child, and to them that give suck in those days! But pray that your flight be not in the winter, neither on the Sabbath day. For there shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be."
The coming of the Son of Man was still future, but the tribulation, mercifully shortened, was past:
Ben.