Right here:
Acts chapter 1
[15] And in those days Peter stood up in the midst of the disciples, and said, (the number of names together were about an hundred and twenty,)
[16] Men and brethren, this scripture must needs have been fulfilled, which the Holy Ghost by the mouth of David spake before concerning Judas, which was guide to them that took Jesus.
[17] For he was numbered with us, and had obtained part of this ministry.
[18] Now this man purchased a field with the reward of iniquity; and falling headlong, he burst asunder in the midst, and all his bowels gushed out.
[19] And it was known unto all the dwellers at Jerusalem; insomuch as that field is called in their proper tongue, Aceldama, that is to say, The field of blood.
[20] For it is written in the book of Psalms, Let his habitation be desolate, and let no man dwell therein: and his bishoprick let another take.
[21] Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us,
[22] Beginning from the baptism of John, unto that same day that he was taken up from us, must one be ordained to be a witness with us of his resurrection.
[23] And they appointed two, Joseph called Barsabas, who was surnamed Justus, and Matthias.
[24] And they prayed, and said, Thou, Lord, which knowest the hearts of all men, shew whether of these two thou hast chosen,
[25] That he may take part of this ministry and apostleship, from which Judas by transgression fell, that he might go to his own place.
[26] And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles.
Matthias was there "ALL THE TIME THAT THE LORD JESUS WENT IN AND OUT AMONG US", with the "us" being the apostles, "BEGINNING WITH THE BAPTISM OF JOHN, UNTO THAT SAME DAY THAT HE", Jesus, "WAS TAKEN UP FROM US", with the "us" being the other apostles.
Got it?
Of course, it's past tense.
Again, Paul said:
I Corinthians chapter 15
[1] Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
[2] By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
[3] For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
[4] And that he was buried, and that he rose again the third day according to the scriptures:
[5] And that he was seen of Cephas, then of the twelve:
After Jesus rose from the dead, he was seen by Cephas or Simon Peter, and then of the twelve (including Matthias because Judas was already dead).
Here is the account:
Luke chapter 24
[33] And they rose up the same hour, and returned to Jerusalem, and found the eleven gathered together, and them that were with them,
[34] Saying, The Lord is risen indeed, and hath appeared to Simon.
[35] And they told what things were done in the way, and how he was known of them in breaking of bread.
[36] And as they thus spake, Jesus himself stood in the midst of them, and saith unto them, Peace be unto you.
The "they" in verse 33 were the two men that Jesus walked with on the road to Emmaus after his resurrection from the dead.
When they returned to Jerusalem, they "found the eleven gathered together, AND THEM THAT WERE WITH THEM" (vs. 33).
Of course, as we've already established from what Peter said in Acts chapter 1, this "them that were with them" INCLUDED MATTHIAS.
Well, what were these two men told when they arrived in Jerusalem?
"The Lord is risen indeed, AND HATH APPEARED TO SIMON" (vs. 34) or Cephas, just like Paul said (I Cor. 15:5).
Now, we're looking for the time when Jesus also appeared to "the twelve" (I Cor. 15:5), with Matthias being the twelfth.
It's right here:
"And as they thus spake, Jesus himself stood in the midst of them" (vs. 36), with the "them" being "the eleven gathered together, AND THEM THAT WERE WITH THEM", and Matthias was there "with them" (Acts 1:21-23).
Got it?