Why would the date "certainly" exist in the database or filesystem? The file might be one file created in 2001 and modified every date after containing every post in it like a mail spool file. It might be a record in a database table with no date column. If it was a single file, then the creation time could be weeks or months before the publication time, and the modification time could be the last time an edit or tweak was made, rendering both useless for this purpose. I think it's more likely not to have a useful date anywhere unless someone went out of their way to put one in.

(Which they should do, it's useful)