That's right. Part of what viewers will see on Monday was filmed almost eight years ago.
"It's surreal that the ending is coming," Thomas said. "We shot that little bit with the kids in fall of 2006, I think, and it's part of the end game. You will see it on March 31."
"How I Met Your Mother" has always been framed as a flashback from the future. Beginning in the first episode, we heard future Ted (Josh Radnor) telling his future kids tales about, yes, how he met their mother.
Video of Ted's teenage children (Lyndsy Fonseca and David Henrie) sitting on a couch in 2030 listening to their father has been interspersed throughout the series.
And those two actors, now in their mid-20s, have "been walking around with the secret all the time," Bays said. When he recently had the opportunity to remind them of that, "Lyndsy was, like, 'What? Oh my God!' She didn't even remember it."
That footage has been "locked up" since 2006 to be incorporated into the series finale. Which may or may not have already been leaked. Or, maybe, longtime fans figured it out.
The thinking is that (POSSIBLE SPOILER ALERT) the mother in "How I Met Your Mother" dies sometime before 2030, and Ted is telling stories about her posthumously.
Bays, Thomas and the cast have been tight-lipped about the theory, although Thomas says the finale will "tie it all together in a package of what this story has been that we've been watching, why we've been watching it. I don't want to say too much about it."
Could he have said too much?
It wouldn't be the first time "How I Met Your Mother" has dealt with death. The series hit a creative high note in Season 6 when Marshall (Jason Segel) learned that his father (Bill Fagerbakke) had died. The show dealt with the loss over the course of several episodes.
Cristin Milioti, who joined the cast at the end of Season 8 as the mother, called the theory "insane" and "crazy" in an interview with The Hollywood Reporter, but she did not deny that it's true.
True or not, "We really did have an eight-year plan from the pilot," Thomas said. An eight-year plan that turned into a nine-year plan, with everything in Season 9 (except flashbacks and flashforwards) taking place on the weekend of Barney (Neil Patrick Harris) and Robin's (Cobie Smulders) wedding a decision that hasn't entirely been embraced by fans and critics.
"There have been pitfalls in it," said Bays, who nonetheless said it was worth it because of the "challenges" it presented the writers.
"We aimed for the idea that a Season 9 episode would feel unlike other seasons that we would give the audience a feeling that something different is happening," Thomas said. "But it's still 'How I Met Your Mother.' "
And sometimes what viewers perceive is not what the writers intended. When some fans took the show as a mystery to be solved, it caught the cast and crew by surprise.
"Around Season 4 people started to think our show was a big puzzle, like a game they were supposed to solve," Harris said, with fans looking for all sorts of clues within the episodes. "They were trying to solve the show, and it was never intended to be something to solve."
As Bays and Thomas have said repeatedly through the years, "How I Met Your Mother" was never really about the mystery of who the mother was. It "was about the audience wanting to spend time with the cast and hearing a life story about the lives of these unbelievably wonderful actors," Thomas said.
The series' nine-year run has turned out to be a major part of not just the careers of stars Radnor, Segel, Cobie Smulders (Robin), Neil Patrick Harris (Barney) and Alyson Hannigan (Lily), but a major part of their lives.
"We watched each other go through, like, some really serious life stuff," Segel said. "People have gotten married and have children. That's a really special thing. As the characters evolved, all of our lives did as well. And some of it paralleled what we've kind of gone through as people. This is the longest I've known any group of people besides my family.
"It's really remarkable. You really do become like family. It's very special."
"I'm just starting to reflect on it and I'm having trouble putting words together," Bays said. "This doesn't feel so much like a phase of our lives as it feels like our lives. Most of our career has been running this show. It's going to be hard to move on."
The one-hour series finale of "How I Met Your Mother" airs Monday at 7 p.m. on CBS/Ch. 2.