As far as I can tell, the forum module itself doesn't have a setting for overriding the time zone. It does appear to be ~20 minutes off for me when I look at it though. We've already set the website time zone and profile time zone. I don't think I've seen any inconsistencies anywhere else. So this might simply be a bug to resolve in the Active Forums module.
I found what may be a related bug reported in the repo:
You may want to report this there as well. Please feel free to reference this forum post when you do.