It seems to me that the date or at least the general time period when the Old Covenant ended would be important to our understanding of eschatology. After all, God’s covenant with the Jews was his modus operandi for about 1400 years, as that pertained to his dealing with mankind. Everything God did in the world, prior to Jesus being born, was either through or as a result of his covenant relationship with the Jews. Even Jesus’ birth was in fulfillment to promises God made to the Jews under that covenant. So, how did the Old Covenant end? Did it end immediately as the new began? Were both in force simultaneously for a period of time? Did it end at all? Is it still in force today, and if so, are both the Old and the New Covenants valid together?
I have been discussing an alleged division in the Olivet Discourse at Matthew 24:36 that some brethren use in an effort to explain away the obvious meaning of the text, which is that the entire discourse describes Jesus’ coming to judge Jerusalem and destroy the Temple (cf. Matthew 23:37-38; 24:1-2). I have been showing how no division exists at Matthew 24:36, and, rather, shows us that the exact time frame of that event was not known during Jesus’ public ministry.
The fact is that Jesus was merely quoting from Zechariah:
And it shall come to pass in that day, that the light shall not be clear, nor dark: But it shall be one day which shall be known to the LORD, not day, nor night: but it shall come to pass, that at evening time it shall be light. (Zechariah 14:6-7)
Notice that Zechariah mentions similar de-creation language that Matthew uses in Matthew 24:29 and 35. The heavenly luminaries aren’t useful on a particular day, and that day is known only to the Lord. That is similar to Jesus’ words, “But of that day and hour knows no man, not the angels, but my Father only” (Matthew 24:36). But, what day is this describing?
The prophet says “in that day” the Lord would make Jerusalem a burden for all nations and they would gather themselves against her, and “in that day” both the warrior and the people would be blinded by the Lord (Zechariah 12:3-4). Even the legitimate authorities would be like a fire that devoured the people (Zechariah 12:6). However, in contrast to this, and speaking of the remnant, “in that day” the Lord would defend the inhabitants of Jerusalem, so that even the weak among them would be courageous and strong (Zechariah 12:8), and “in that day” would be a time of reward of the righteous and punishment of the wicked (Zechariah 12:9, 11; 13:1-2, 4).
The Lord would gather all nations against Jerusalem and the city would be taken. Half the city would be taken captive and the other half slain (Zechariah 14:2), and “in that day” the Lord would descend from heaven and he would stand on Mount Olives, which would cleave in the midst, creating a valley toward the east, through which some would flee (Zechariah 14:4-5). This language is similar to what Josephus said occurred, when the Roman general, Cestius, took the city at the beginning of the Jewish war with Rome, but the left the city. The Christians, remembering Jesus’ words:
And when ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh. Then let them which are in Judaea flee to the mountains; and let them which are in the midst of it depart out; and let not them that are in the countries enter thereinto. For these be the days of vengeance, that all things which are written may be fulfilled. (Luke 21:20-22; emphasis mine)
It seems very clear that Zechariah 12 through 14 is speaking of the destruction of Jerusalem prophesied to occur in 70 AD. Jesus also pointed to these same events in the Olivet Discourse, and Jesus’ words, “But of that day and hour knows no man… but my Father only” is an echo of Zechariah’s “it shall be one day which shall be known to the LORD” (Zechariah 14:7), which, according to Zechariah describes the day Jerusalem was prophesied to fall, which is the day that Luke describes as the days of vengeance when all things there were written would be fulfilled. That would be when the Old Covenant ended—when all things that were written would be fulfilled (cf. Matthew 5:17-18).
 Josephus; Wars 2.19.4-7 & 2.20.1