I prefer to see absolute time - if I'm commenting somewhere online, I'm probably not in tip-top cognitive shape and I usually check only every so often.
> Both are not hard to implement. But exact timestamp has a timezone problem
Specify a timezone in the timestamp. UTC is probably fine, especially if you are doing both, in which case, the main value of timestamps (AFAICT) is to compare the relative times of other posts that are hard to determine from the prose descriptions.
Brilliant! I've been using timeago (http://timeago.yarp.com), but Moment.js (especially with its timezone support) may solve some of my problems better.
Set it up so the date is clickable and clicking it toggles the mode. Also save the current mode to a preferences json object in localstorage for persistence.