Facebook has users all over the world but they display times in everything in local times. Shouldn't be that hard. Having memes and comments marked with hours elapsed is dumb I think.
Facebook is not unique there. Some sites can even track your location to fill in the correct time zone (although this routine has fallen a bit in popularity due to privacy issues). "Hard" is the wrong word... It can only provide a lot of work and I know out of experience that bugs can easily pop up. It's for that reason why loads of sites have now chosen for a "5h ago" system (which you can also see on imgflip) in order to avoid time zone issues.
It's a decent QOL suggestion, though a tiny bit redundant for comment notifications, since comments already tell you how old they are, at least to the nearest hour.
