I take my doctrine from authoritative NT apostolic teaching, not from personal interpretation of prophetic riddles not spoken clearly (Nu 12:8), which interpretation is contrary to that apostolic teaching.
It's not about you referring to it, it's about what it declares regarding the nature of the source of your doctrine; i.e., that it is riddle and not spoken clearly, which riddles you interpret contrary to authoritative NT apostolic teaching.
The only problem here is you are taking a verse and indeed the phrase "last day" in that verse of John 6:39 at the expense of other scriptures for which you cannot align with your belief in how you are applying last day to mean.
That is not rightly dividing the word of truth and therefore you are in need to ask Jesus Christ for wisdom.
So ask Him for wisdom to see the truth in His words because it sure does not hurt to ask Him to make sure you are not in error as overlooking the truth in His words in other scripture that shows you are not rightly dividing the word of truth about that "last day" in John 6:39.
Proof? Look at John 6:37 then.
John 6:37 All that the Father giveth me shall come to me; and him that cometh to me
I will in no wise cast out. 38 For I came down from heaven, not to do mine own will, but the will of him that sent me. 39 And this is the Father's will which hath sent me, that of all which he hath given me
I should lose nothing, but should raise it up again at the last day.
We have Him saying any disciple that stops abiding in him will be cut off per verse 6 of John 15:1-8.
Luke 12:40-49 testify to saved believers being cut off and ;left behind with unbelievers to face that fire coming on the earth per verse 49. AND in verses 46-48 is about those servants left behind that are getting stripes per the knowledge they had for not being ready & lesser stripes for the knowledge that they did not have for not being ready, but stripes they will get.
Hebrews 12:1-10 talks about running that race and those who look to Jesus Christ daily in helping them lay aside every weight & sin are running that race whereas those that do not, are not even resisting sin, they are going to get a scourging; hence stripes from the Father hence tying in with that pre great tribulation rapture event when those left behind will get those stripes so that hey may be partakers of His holiness.
Matthew 5:19
Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
Now consider teaching any one to break any commandment while you do the same; ridiculous, right? And yet you are considered the least in the kingdom of heaven. How can that be?
Jesus is talking about those who commit iniquity that gets left behind, with stripes, will become vessels unto dishonor in His House as vessels of wood & earth but in His House. That is why Paul is calling every believer that has His seal and that foundation laid by Jesus Christ to depart from iniquity, even former believers are still called to go to Jesus for help to see the truth to depart from iniquity, and so that is why YOU CANNOT apply that as being addressing the world when Paul is warning believers to be ready by departing from iniquity pr else be left behind to get stripes because like it or not, those resurrected after the great tribulation is going to have those stripes to remind them severely not to commit those sins again as they will not be teaching others to break any of His commandments then but this explains how they winded up as the least in the kingdom of heaven.
2 Timothy 2:18 Who concerning the truth have erred, saying that the resurrection is past already; and overthrow the faith of some. 19 Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, let every one that nameth the name of Christ depart from iniquity. 20 But in a great house there are not only vessels of gold and of silver, but also of wood and of earth; and some to honour, and some to dishonour. 21 If a man therefore purge himself from these, he shall be a vessel unto honour, sanctified, and meet for the master's use, and prepared unto every good work.