The date of the Purge is December 19, 1992. This is based on December 19 being Ben's birthday (from the reasoning for the Lostpedia timeline) and the purge happened on his birthday. It was in 1992 because Horace said, in a vision, that he had been dead for 12 years (Cabin Fever). The date of the Purge is December 19, 1988. The dream encounter between Horace and Locke could have also taken place in 2000. The vision is pretty inconclusive, especially given the fact that we see Horace working on his cabin which took place before 1992, and Mikhail's statements can't be taken at face value.