Unfortunately, I don't think this is something that can be changed on a per-user basis. I didn't like that style either at first, but I now understand why the software developers chose to do it that way: As we've got members from all over the globe, for users that aren't logged in we won't know their time zone (so we use GMT). This means that the times would look really strange for most site visitors, but showing something like "1 day ago" or "20 mins" ago is a time-zone agnostic way of doing it.
I'll have a quick look at seeing if it's possible to make a user option for this, just in case it's easy enough to code - but no promises on this one, as I think it's probably more complicated to change than I imagine

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