Only a mug punter myself with the search process. However, one needs to be aware that a search can only return a maximum number of hits (500, as I recall). If your subject has more than this then you have to be a bit sneaky and set up back to back searches .. eg,
(a) always use the advanced search option. Then, you might do
(b) one search at one year and later, with a second at one year and earlier, with all forums selected, or
(c) do something similar, but with restricted forum, or groups of forums, applicability.
(d) try a concatenated search using both keyword and username. Keep in mind that you have an option for searching title or post and posts or threads.
Using some of the tricks above lets you get back to the early dates.
Keep in mind that there were some monumental crashes in the early days and older threads may be lost forever.