Apart from 'test it works', your usual reminder: if you're tempted to exclusively present relative times, don't.

(This advice covers a most cases. There are a couple of exceptions, but they are so specific that almost all off-the-shelf attempts at doing relative time will, in some way, be wrong.)

Well, while I'm here ranting about it, Meetup is an example of the kind of site you get when you lack an overall design concept (both for the UX and for the architecture).

It also shows the consequences of not having good automated tests: looking at a comment posted yesterday, which is listed as having been posted 57 days ago, and one a couple of weeks ago as having been posted 86 days ago.