As per the current WebVTT specification, changes to the displayed text should only apply when the CSS :past and :future pseudo-classes are used. Otherwise, the text displayed should be the same as without timestamps.
Have we implemented it differently? Maybe what we have makes more sense and we should see that WebVTT is changed?
(In reply to comment #1) > Have we implemented it differently? Maybe what we have makes more sense and we should see that WebVTT is changed? Yes, it's different in the sense that we hide by default the future nodes, while the spec doesn't. The spec certainly needs some changes and maybe we should file a spec bug for implicitly hiding future nodes (I don't see any why an author would specify timestamps and not want the future nodes to be hidden). I've been pointing out some of the changes required here: http://lists.w3.org/Archives/Public/public-texttracks/2012Sep/0069.html
I suggest registering a bug on the spec and not making any changes here, since I agree that our behaviour makes a lot more sense.