Stephen Colbert and Ryan Reynolds have a time-traveling argument on 'The Late Show'

We'll let you in on a little secret about late-night TV: Almost all of it is pre-taped. Yes, there's a studio audience and and the host's jokes are timely so it looks like it's all happening as you're watching, but the fact is almost all the shows are taped in the afternoon before airing. And in some cases segments are done days before.

That was the case with Ryan Reynold's appearance Tuesday on "The Late Show with Stephen Colbert." Unable to be in New York City this week to promote his new movie, "Life" (opening Friday), he recorded his appearance last Thursday. But Colbert and Reynolds had some fun with it. 

Instead of playing it straight, Colbert did a bit in which Reynolds comes out in the middle of Colbert's monologue and argues with him over what day it is.

"I can't stand here and let you lie to this audience," Reynolds says as Colbert, looking puzzled, is in the middle of saying that Tuesday is actor Matthew Broderick's birthday, and he's "one of America's finest actors."

"You don't think Matthew Broderick is one of America's finest actors?" Colbert asks Reynolds. 

"Of course he is, Stephen, he's a national treasure," Reynolds says. "But I've been sitting back there listening to you deceive these people. Today is not Tuesday - it's Thursday. You pre-tape this entire show!"

Colbert is outraged by the accusation and proves that it's Tuesday by delivering a timely joke: "So March Madness is upon us, and did you hear this? There's only 16 teams left in the tournament."

"That's how it's scheduled, there's only 16 teams by now," Reynolds says.

Reynolds then presents a newspaper dated Thursday, March 16. Colbert counters, saying Reynolds could have just held onto that paper. The host then brings out a paper from 1912, with a headline showing that the Titanic has sunk.

The two look to be at a stalemate until Colbert delivers the coup de grâce: a taco.

"Can't have tacos on Thursday," the host says, referring to the American tradition of many restaurants having special offers on tacos on Tuesdays.

Reynolds finally gives in and agrees that it's Tuesday, as he devours the taco.

Watch the entire bit below: