XF 1.5 Google results displaying old date on forum category pages

[PetForums]

Hi All, just wondering if anyone has any idea why google is displaying this old date "9 Apr 2017" against my forum category pages when they appear in the google search results. I noticed it on category pages only and not on the homepage. The forum is a busy forum and updated regularly. I'm not sure where the date is coming from or how to change it. If its not something that can be easily changed, how would one provide the date of the latest post in that category somewhere on the page that google could possibly pickup?

see image attached.

 

[Bimble]

I expect that's the date Google last indexed it, and whilst the threads will change regularly, the main structure of the page won't.

 

[PetForums]

I expect that's the date Google last indexed it, and whilst the threads will change regularly, the main structure of the page won't.

I thought that, but checking googles cached version of the pages, shows that they where last cached by google yesterday. Google obviously thinks that the "9 Apr 2017" is an important date for the pages, but I cannot see any reference to that date in the source code on any of the pages.