I'm not very knowledgable about the subject, but I don't think Time is abstract. I mean, we face it and deal with it and think about it all the time. See, even me saying that involves using the word 'time' because it is a large basis for our society. Time is linear because that's just how it works from our perspective, although maybe not from others like the red shift or looking at faraway galaxies, thats a different perspective.
It could be abstract, but since we deal with it so much we've become so accustomed to it that it's engraved in us in absolute terms, like I've got a class at 11:30 or meet you at 7 tonight, etc.