Yes, the internal search is "iffy" at best...
I manually found a post made in Sept 2004 (just a test case, nothing special) and using WP's search I can find posts made by the author right back to 2004 to the date they joined - but if I include a search term, a couple of words from the sample post, nothing is found.
If I remove the author's name from the search, leaving only the search term, results are returned up to 2014/15 only.
I get much better results using a Google search, external to WP.
Instead of using Google with just search term, which will return absolutely everything it can find worldwide with search term, you can tell it to search a specific site by adding information to the search string, as: site:wrongplanet.net search term
The results will now only be from WP.
Better, you can define a search time range to further filter the results.
Underneath the Google search bar, to the right, is a button labeled "Tools".
Click on that and select "Any time" (it's a drop-down menu) then select "Custom range". You can now enter a From and To date range.
There's a further drop-down, "All results", from which you can select "Verbatim" as an additional search term delimiter.
The results will now better reflect what's publically available on WP to an external search engine - but this won't of course include results from the adult and members only forums because they're not publically accessible unless you're a member, and have added a birthdate to your profile.
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Giraffe: a ruminant with a view.