First of all I wonder what the point is of posting a topic with the date reflecting the document, the document is old but posted recently. Nothing special just specify in the post its date of origin. However, if you really want you can change the start date of your board with this extension
https://www.phpbb.com/customise/db/exte ... ine_since/ at least you won't find problems if someone searches by date. I've always been against changing dates in posts because searches and all the code is based primarily on post IDs. It's a basic subterfuge that will lead to other problems in the future. Your choice.