I'd answer this by pointing out that modals are 'helping' verbs - they help us to understand a given concept, one of which might indeed be time. Others are intent, possibility, probability and so on. As for your argument, 'they do not show time' I am not sure I would agree with you entirely, but i'll admit they need a little help sometimes: 'I couldn't find a hotel room in London!' This is either lack of ability in the past (add 'yesterday as a complement) or future inability (add it's beyond me). I'd be interested to hear what 'actions' you think modal verb show :)

When you learn modal verbs it's better if you focus on the use that each one has e.g. You use can to show ability, may to show possibility etc. They show time but it depends on the structure e.g. I might go to the cinema tonight (future), He might have forgotten his appointment that's why he didn't come. (modal perfect for past)