We go back to when Desmond and Sayid took off in the helicopter with the non-drunk Nick Nolte to head back to the freighter. They run into some turbulence, not helped by the fact Desmond wigs out while flashbacking to his stint in the Scottish army in 1996. What"s strange is that Desmond"s consciousness seems to be switching back and forth between the two Desmonds, and it"s Military Desmond who is the real Desmond, and he thinks Island Desmond is a dream. Island Desmond doesn"t recognize Sayid or even know where he is.
Sayid"s confused because they left the island at dusk but arrived in the middle of the day after only a short flight. He calls Jack on the island, where it"s been almost two days since the helicopter left. Daniel and Charlotte (especially Charlotte) are really cagey about what"s going on, but Daniel tells Island Desmond via the satellite phone that the next time he switches back to Military Desmond, to look him up at Oxford, and to tell him to set the machine for 2.343 and to oscillate at 11 megahertz. And if that doesn"t work, Military Desmond is to say, "I know about Eloise." See, sometimes people can get quite confused on their way to or from the island, especially if they"ve had some exposure to electromagnetism.
Military Desmond does as he"s told, and finds younger Oxford Daniel is quite the professorial douchebag. Nevertheless, after passing on the message, Daniel thinks Desmond might very well be from the future, and, using the settings, points his machine at a little mouse named Eloise. The numbers Desmond provide get Eloise"s consciousness "unstuck" in time, and allows her to quickly run a complicated maze that Daniel hasn"t taught her to run yet. I think it"s a bit of a leap to go from "mouse quickly runs maze" to "my time-travel experiment is a success!" Especially when Eloise the mouse dies of a brain aneurysm before too long. Daniel figures flipping back and forth between two time periods is plenty tough on the old grey matter, so he tells Military Desmond that Island Desmond better have an anchor (a metaphorical one, as opposed to the freighter"s anchor).
So in 1996, Desmond harasses ex-girlfriend Penny (after meeting with Mr. Widmore at an auction that drops a ton of episode callbacks) into giving him her new phone number. He promises he won"t call her today, or tomorrow, but in eight years, on Dec. 24, 2004. Penny, pissed that Desmond"s calling her today, apparently thinks, "Right, I hope he"s STILL STALKING ME EIGHT YEARS FROM NOW," and gives him her phone number. He"s going to need that, if he"s to avoid winding up like Minkowski, whom we finally meet on the freighter, not long before he shuffles off this mortal coil after flipping back and forth one too many times.
As we know, things have changed between Desmond and Penny in the intervening eight years, and she"s thrilled to hear from him, and they have a tearful (and, to be honest, quite tear-jerking) conversation, despite Desmond looking more and more like Jimmy Fallon as Barry Effin" Gibb.
So: time-travel. Perhaps Desmond in 2027 should be sending a T-1000 to the island to look after the Lostaways.