Here are a few possibilities:
1: It’s simply a reassuring figure of speech, to let the thief know he had been saved by his honoring of Christ and show of humility.
2: Paradise in this case refers to Abraham’s Bosom, which is where the souls of the righteous dead went before the gate of Heaven were reopened to them. The thief went there with Jesus, and then went to Heaven with all the other righteous dead. He probably need a topping of in Purgatory, though it is entirely within God’s power to make that cleansing instantaneous.
3: The Time of Heaven is not the same as the Time of our physical existence. We know that we will have same concept of “time” in the afterlife, but we do not know how that time relates to the time we know now. It may be that Jesus did not intend a 24-hour block of time when He said “day”.
Personally, I lean towards options 1 and 2. A lot of people take every word Jesus said as being 100% literal, and that just doesn’t make sense. He uses parables and idioms throughout His ministry, and I’ve always just assumed this is another such instance. The point is that Jesus wanted the thief to know that he was saved for his show of faith and humility.
Honestly, I kinda think it was much more for our benefit than for the thief’s. I don’t believe that man was there on accident. God wanted to show us that it doesn’t matter how late you find your faith, you can always be saved if you turn to God with a repentant heart.