Dwindling in Unbelief
-God's Killings in Genesis
The phrase "truly I say unto you" is used by Jesus more than 50 times in the NT. In all verses except this one, the NWT places the comma after the word "you". But the NWT translates this verse as follows: "Truly I tell you today, You will be with me in paradise."
Why would they do that? Well, the Governing Body doesn't believe in a soul, so they say the good thief died like everyone else. Only later, nearly 2000 years later, after Armageddon, would he be resurrected. So they couldn't let Jesus tell him that he'd be with him that very day in paradise. Instead, they have Jesus say that he is telling the truth today (Not yesterday or tomorrow, but today. Or maybe "today" here is emphasizing that he's telling the truth today, but he was lying yesterday or something) -- and that sometime in the far distant future they'd be together in paradise.