I have noticed that, at least Reddit, is drastically inaccurate with the dates - Google Search has the same issue, FWIW. I'm not sure where either of them are pulling the date from - I can't even see correlation with comment dates. This however, does make searching for a date range (aka up-to-date information) a bit useless as it is.
I suspect hidden comments (aka spam comments hidden automatically) are to blame - but that's impossible to figure out.