ABC’s Lost show secretly hinted at its time travel twist long before it was officially implemented into the story. As a plot device typically associated with the science fiction genre, time travel didn’t originally fit into Lost's central mystery. That being said, it ended up serving as the driving force behind the story for Lost season 5.
At first, Lost’s exploration of the time travel concept was limited to its Desmond-centric episodes, which took Henry Ian Cusick’s character down a bizarre path that differed greatly from the experiences of the other characters. But after Ben (Michael Emerson) succeeded in “moving” the island in the season 4 finale, Desmond wasn’t the show’s only time traveler anymore. As a result of Ben’s actions, the main characters began leaping forward and backward through time throughout Lost season 5. Much of the story was set in the 1970s, where Sawyer (Josh Holloway) and the others made homes for themselves at the Dharma Initiative.
A scene in Lost season 2 proved that time travel being added to the mix didn’t come out of nowhere. In fact, it was teased in a conversation between Hurley (Jorge Garcia) and Sayid (Naveen Andrews) in Lost season 2, episode 13, titled “The Long Con”. While listening to a radio, the two heard a clear broadcast of a classic Glenn Miller song from the 1940s. Sayid said it could be coming from anywhere, which led to Hurley joking that it could actually be coming from “any time”. In a recap episode that aired before the season 5 finale, producers Damon Lindelof and Carlton Cuse confirmed that Sayid and Hurley really did unknowingly intercept a radio broadcast from the 1940s.
As it turned out, time travel was at play as early as Lost season 2. It would seem that the electromagnetic energy stored within the island that made Desmond so special and sent the main characters to the past also caused other time-related anomalies. Apparently, Hurley and Sayid hearing a song from a 1940s radio station was one of them. Though it was subtle, it was the earliest indicator that Lost would eventually make time travel a factor. A different reference was almost made in Lost season 1, but it was cut from the script [via Lostpedia].
What all this means is that time travel was a part of the creators and writers’ plans for the show since the early days of the series, if not the very beginning. Not only did the concept play an important role in the progression of events, but it set the stage for a number of big reveals. It was through the characters’ trips to the past that Lost was able to answer a number of major and minor mysteries connected to the Smoke Monster, the Dharma Initiative, Danielle’s research team, the Others, and more.