The problem isn't a technical one related to timezones. The problem is that full date/timestamps are less useful for a reader to see than relative dates in many cases.

Using relative dates is basically saying, "Our old stuff has no worth to you. Please never care about it."

How?

Explain why relative dates are more useful. You'll see it.

But if a user sees a post on someone else's feed, do they know if the timezone is adjusted or not?