The Lost episode in which Desmond Hume repeatedly experiences moments across different times is “The Constant.”
“The Constant” is the fifth episode of the series’ fourth season. During a helicopter journey, Desmond’s consciousness begins shifting between 1996 and 2004. The shifts leave him disoriented and create a deadly risk unless he can establish a mental connection between the two periods.
With help from Daniel Faraday, Desmond searches for a person who can anchor him in both times. That person is Penny Widmore, whose emotional importance gives the episode its title and its resolution. The story expands Lost’s treatment of time travel without presenting the episode as a conventional flashback.
The episode is often remembered alongside “Flashes Before Your Eyes,” another Desmond-centered story involving unusual time experiences. “The Constant,” however, specifically focuses on Desmond’s shifting consciousness and his need to find an anchor.