Best I remember, there was no date in June officially cited, at first, for LP going missing -- just the month of June. Later, LT said LP went missing same night as the accident -- and still later also added LP was murdered the night of the accident (but before the accident).
It seems like an "official" date of death for Lisa was at first the date in Sept. 1999 when her body was found, because a poster found a Social Security death record or something similar with that date. It appears the family/and or LE went with the death date of July 3 at some point later.
Screen shot from ancestry.com taken on July 16, 2012. The family tree seems to be created by one of Lisa's siblings.