In the Concordant Literal New Testament we read:
Because either is grammatically possible, the question of where the comma should be placed when translating the verse into English must be determined by other considerations. Christ's words, "Truly, I say to you, this day you shall be with me in paradise," may just as legitimately be constructed as, "Truly, I say to you this day, you shall be with me in paradise." By placing the comma after "today" instead of "you," the fulfillment of Christ's promise need not be confined to the day in which they died, but may be understood as having its fulfillment at some time in the distant future. This translation is not only grammatically valid, but it makes Christ's words consistent with what the rest of Scripture teaches (as well as what Christ himself said) concerning the state of the dead.