Times and dates not showing on forum

[Dplax]

Hi all, I upgraded my forum from 4.2.5 to 5.4.3 and I seem to have lost the time and date in doing so - they are showing up as d-m-y and H:i on the front end. No numbers, just the letters like that. This is displayed on post dates and other places. I have tried rebuilding all the indexes/running the maintenance tasks, but no luck.

Any ideas as to what could cause this? It's a pretty big issue. Thanks.