Scratch that, saw this post by Cayley and it put the time-travel thing in better perspective...
I found this at Lostpedia, but it doesn't say where or by whom these dates are confirmed:
The various time shifts have been dated:
* The first time shift occurs in the late 1990s, a few years before the plane crash, during Mr. Eko's adult life in Nigeria and Ethan Rom's on the island. However, time behaving differently on the Island compared to the outside world can cause some changes.
* The second shift is January 2005 or later, as Richard Alpert finds Locke and tells him that the Oceanic Six are already home.
* The third shift happens between 2001 (Desmond crashes onto the island), and September 22, 2004 (Kelvin dies).