this sounds to me like a pattern or basically there is more to it than just him tracking when you get off the computer. (is there an issue from your past that has something to do with you being up late on the computer? maybe not, just a thought.)
yes some parents here have said as long as you're in his house you must follow his rules, but there is something to be said for a conversation (being calm will often lead to more progress with someone). explain how you want to understand the reasons for this. when you talk with him, focus on saying how it feels to you and what you think about it, instead of talking to him about him.
maybe he doesn't like the fact that you're growing up. or maybe he doesn't want you looking at pornography. i don't know, but i think it is returning manipulation with more manipulation for you to just try and find a way around his tracking your time on the computer. if you want to be seen as mature enough to choose your own bedtime, i think you are better off having a discussion. but that may not be what you want. so hopefully you can use some of the tech advice offered already.
along the lines of the research idea already offered, you can allow your computer to be used for another ambitious project and just leave it on all night: SETI@home
if your computer has an energy saving mode, you might be able to have it on constantly and then there would be no power off time. not a very eco-friendly option but you may not care about that.