Again, the part that TE's are missing is the allegory the early fathers believed in. They believed that the days of Genesis were literal morning evening days, and that they also had allegorical information about the future. For instance,
The Sabbath is mentioned at the beginning of the creation [thus]: “And God made in six days the works of His hands, and made an end on the seventh day, and rested on it, and sanctified it.” Attend, my children, to the meaning of this expression, “He finished in six days.” This implies that the Lord will finish all things in six thousand years, for a day is with Him a thousand years. And He Himself testifies, saying, “Behold, to-day will be as a thousand years” (Psa. 90:4; 2Pe. 3:8). Therefore, my children, in six days, that is, in six thousand years, all things will be finished. “And He rested on the seventh day.” This means: when His Son, coming [again], shall destroy the time of the wicked man, and judge the ungodly, and change the sun, and the moon, and the stars, then shall He truly rest on the seventh day. (Epistle of Barnabas, II.15)
Notice how Irenaeus puts it.
For in as many days as this world was made, in so many thousand years shall it be concluded. And for this reason the Scripture says: “Thus the heaven and the earth were finished, and all their adornment. And God brought to a conclusion upon the sixth day the works that He had made; and God rested upon the seventh day from all His works” (Gen. 2:2). This is an account of the things formerly created, as also it is a prophecy of what is to come. For the day of the Lord is as a thousand years (2 Pe. 3:8); and in six days created things were completed: it is evident, therefore, that they will come to an end at the sixth thousand year. (Irenaeus, Against Heresies, V.28.3)
Notice he said these literal days
also represented prophecies of 1000 years in the future. Each day represented a future 1000 years. Guess what? Irenaeus was a young earth 6 day creationist. Now he believed the literal days represented futures periods of 1 thousand years each. But the fact that he believed that after 6 thousand years the millennium would come shows how literally he took the Genesis days to be.
But that we may not leave our subject at this point undemonstrated, we are obliged to discuss the matter of the times, of which a man should not speak hastily, because they are a light to him. For as the times are noted from the foundation of the world, and reckoned from Adam, they set clearly before us the matter with which our inquiry deals. For the first appearance of our Lord in the flesh took place in Bethlehem, under Augustus, in the year 5500; and He suffered in the thirty-third year. And 6,000 years must be accomplished, in order that the Sabbath may come, the rest, the holy day “on which God rested from all His works.” For the Sabbath is the type and emblem of the future kingdom of the saints, when they “shall reign with Christ,” when He comes from heaven, as John says in his Apocalypse: for “a day with the Lord is as a thousand years” (Psa. 90:4). Since, then, in six days God made all things, it follows that 6,000 years must be fulfilled. And they are not yet fulfilled, as John says: “five are fallen; one is,” that is, the sixth; “the other is not yet come” (Rev. 17:10). (Hippolytus, Exegetical Fragments on Daniel, II.4)
Above, Hippolytus also makes the same point that there must be 6000 future years after creation before the time of rest, since creation itself took 6 literal days prior to God resting. A genuine YEC 6 literal day creationist.
Methodius makes the same point:
For since in six days God made the heaven and the earth, and finished the whole world, and rested on the seventh day from all His works which He had made, and blessed the seventh day and sanctified it (Gen. 2:1), so by a figure in the seventh month, when the fruits of the earth have been gathered in, we are commanded to keep the feast to the Lord, which signifies that, when this world shall be terminated at the seventh thousand years, when God shall have completed the world, He shall rejoice in us (Psa. 104:31). (Methodius, The Banquet of the Ten Virgins, IV.9)
Again, 7000 years for 7 days. He believe in literal creation days, and that only 7000 years would span after creation.
Therefore let the philosophers, who enumerate thousands of ages from the beginning of the world, know that the six thousandth year is not yet completed, and that when this number is completed the consummation must take place, and the condition of human affairs be remodeled for the better, the proof of which must first be related, that the matter itself may be plain. God completed the world and this admirable work of nature in the space of six days, as is contained in the secrets of Holy Scripture, and consecrated the seventh day, on which He had rested from His works. But this is the Sabbath-day, which in the language of the Hebrews received its name from the number, whence the seventh is the legitimate and complete number. For there are seven days, by the revolutions of which in order the circles of years are made up; and there are seven stars which do not set, and seven luminaries which are called planets,whose differing and unequal movements are believed to cause the varieties of circumstances and times. (Lactantius, Divine Institutes, VII.14)
Lactantius above says the same thing. Since God created everything in 6 days, there must be 6000 future years before God's rest comes. He explicitly says those years had not yet been fulfilled at the time of him writing this. Thus he was a YEC as the majority of the early fathers were.
To me, as I meditate and consider in my mind concerning the creation of this world in which we are kept enclosed, even such is the rapidity of that creation; as is contained in the book of Moses, which he wrote about its creation, and which is called Genesis. God produced that entire mass for the adornment of His majesty in six days; on the seventh to which He consecrated it . . . with a blessing. For this reason, therefore, because in the septenary number of days both heavenly and earthly things are ordered, in place of the beginning I will consider of this seventh day after the principle of all matters pertaining to the number of seven; and as far as I shall be able, I will endeavor to portray the day of the divine power to that consummation. . . . And in Matthew we read, that it is written Isaiah also and the rest of his colleagues broke the Sabbath (Mat. 12:5) – that that true and just Sabbath should be observed in the seventh millenary of years. Wherefore to those seven days the Lord attributed to each a thousand years; for thus went the warning: “In Thine eyes, O Lord, a thousand years are as one day” (Psa. 90:4). Therefore in the eyes of the Lord each thousand of years is ordained, for I find that the Lord’s eyes are seven (Zec. 4:10). Wherefore, as I have narrated, that true Sabbath will be in the seventh millenary of years, when Christ with His elect shall reign. Moreover, the seven heavens agree with those days; for thus we are warned: “By the word of the Lord were the heavens made, and all the powers of them by the spirit of His mouth. (Victorinus, On the Creation of the World)
Victorinus also agrees. Each of the days of creation stood for future years to come, precisely 7 thousand years, since there the creation consisted of 7 literal days.
Again, what TE's don't understand the theology the early fathers and the type of allegory they embraced. They weren't denying the historical narrative for the same of modern theories, in fact, they thought such methods were erroneous. Rather, they added allegorical predictions to the literal days. They were nowhere near the TE's of today and their erroneous methods of interpretation.
And who knows, maybe some of their future allegorical predictions will turn out to be true. The raptures may be right around the corner.