Of course punctuation did not appear in the earliest Greek texts so the punctuation must be added by the translators.
Many translations have placed the comma AFTER “today” rather than in front of it and so read "I tell you today, you will be with me in Paradise."
Obviously these translators felt that the word SHMERON (today) had reference to the time the promise was made rather than to the time it was to be fulfilled. Their choice was solidly based on valid translation principles including Greek grammar, usage and context.
While scholars generally view the grammar as allowing the comma to occur in either position, when we look at identical grammatical constructions we find the grammatical evidence strongly argues for the comma to be placed after the word "today."
For example, if Luke had wanted to say "today you will be with me" instead of "I tell you today," he could have used hOTI, (that) before "today" (Lk 2:11; 4:21; 5:26; 19:9; 22:61).
On the other hand, Luke could also have made it clear that he meant to say "I tell you today, you..." by placing HOTI after "today." Since he did not use HOTI at all the matter cannot be *absolutely* settled by grammar alone.
But, we also have strong evidence which supports placing comma after “today” from the use of the exact phrase in the OT.
There are at least 33 occurrences of "I tell you today" in the LXX which are grammatically parallel to Lk 23:43 in using a verb of speech or command with "today." ALL these examples are clearly to be understood with SHMERON (today) connected with the preceding verb so as to stress the veracity and significance of what is being said. That is, the comma should follow "I tell you today, ..." in every case.
Lamsa, in "Gospel Light" says: "According to the Aramaic manner of speech, the emphasis in this text is on the word 'today' and should read: 'Truly I say to you today, you will be with me in Paradise.' The promise was made on that day and it was to be fulfilled later. This is characteristic of Oriental speech implying that the promise was made on a certain day and would surely be kept."
So, the overwhelming evidence in the occurrences of this exact phrase is that Christ's use of the word "today" was not to give the time of the evildoer's being in Paradise but, rather, to call attention to the time in which the promise was being made.
Now, although the weight of evidence from grammar and usage resoundingly supports placing the comma after “today”, the definitive proof comes from the context.
First, Mark himself and other Scriptures state that Jesus was dead and in the grave (hADES) for three days. So he could not have been in "paradise" on that day (Lk 24:46; 9:22; Ac.2:31; Mt.17:23; 1Cor.15:4). The Bible states explicitly that those in hADES are not conscious (Gen.3:19; Eccl.9:5,10; Ps.146:4).
In "How To Choose a Bible Translation" J. Parkinson says: "‘And Jesus said unto him. Verily I say unto thee today, thou shalt be with me in paradise.' Grammatically the comma goes equally well before or after the word ‘today'...It is preferred to go after ‘today,' because until the third day, according to Acts 2:31, Christ went to ‘hell'--which is not normally considered Paradise."
Putting the comma before the word "today" would make Jesus a liar since he knew he would be in hADES for three days not in "paradise." Putting the comma after the word "today" causes no such contradictions.
Second, Jesus had clearly stated that his faithful disciples would be with him in his kingdom only in the distant future (Lk 22:28-30; Jn.14:1-3; Mt 19:28). If the long-faithful Apostles will not be with him until his far distant future coming, why would the evildoer get to be with him immediately?
Third, Scriptures plainly state that the resurrection of chosen humans to eternal life with Christ would only occur after Christ's future coming and presence in the "last days." God did not promise anyone that they would be rewarded at death. (Mt.16:27; 24:3; Lk.14:14; Jn.11:24; 14:3; 1Cor.15:22,23,51,52; 2Tim.4:8)
Many years after Christ's resurrection the righteous dead were still "asleep" in death (1Cor.15:6; 1Thes.4:13) and the resurrection was still future (Ac 24:15; Rev. 20:12,13; cf. 2Tim.2:18).
Since Jesus was "the first fruits" of all resurrected to eternal heavenly life the man could not have preceded him to heaven that very day (Ac.10:40; 1Cor.15:20,23; Col.1:18).
According to all these explicit passages the resurrection of the thief could not have taken place on the day of his death.