If you download a Data export, the timestamps are there for every conversation, and often for messages as well.

The html file is just a big JSON with some JS rendering, so I wrote this bash script which adds the timestamp before the conversation title:

  sed -i 's|"<h4>" + conversation.title + "</h4>"|"<h4>" + new Date(conversation.create_time*1000).toISOString().slice(0, 10) + " @ " + conversation.title + "</h4>"|' chat.html