Revealing it on hover would be the desktop UI answer and perfectly achievable on the web if they wanted to. Facebook does this on post times. I think it's a decent tradeoff.

Unfortunately mobile doesn't always work nicely with hover semantics.

Many sites also put it in the title text (tooltip, i.e., hover / long press). When that fails, I often find it by inspecting the element and looking at every attribute of the elements within 1 level on the tree. Horrible experience.