Time is often abbreviated in German (like in English, a.f.a.i.k.). "Wir treffen uns um acht." = "Wir treffen und um 8 Uhr" (Let's meet at eight. = Let's meet at 8 o'clock.)

I'm not sure what you mean but you might mean this: Stunde (plural: Stunden) vs. Uhr 8 Stunden = duration of 8 hours 8 Uhr = 8:00 (time) So if you want to say that you've learned for 2 hours, you have to say that you've learned for 2 Stunden. But if you want to say that you've learned from 2:00 till 4:00 you have to say '2 Uhr bis 4 Uhr'.