Time in Mandarin, from what I remember (and I would urge you to check what I'm saying, as I haven't spoken Mandarin much in the past year, though I did live in China for two years before that), is a function of where it comes in the sentence. So, for example, "We start class at 10:00" would be expressed like this (this computer can't do Chinese characters): Shi dian wo men shang ke. If you're talking about the duration of something, you put the time after the verb or verb phrase. Wo men you ke shi xiao shi. There aren't any prepositions, unless you start talking about before, after, etc.

What specifically are you trying to express? When something takes place? Duration?