Help me with this riddle.

You sure that's the right answer? Doesn't seem to address the fact that:

Well it does. I could pick up a clock and change the time on it.

Why "certain" then? Couldn't anyone adjust a clock?

Well, I wouldn't let you touch my fucking clock. I'd tell you to put that shit down.

I'm pretty damn sure riddles would have long lost their appeal if everyone approached them with that attitude. Ignoring half the riddle or deliberately misinterpreting it just isn't the right way to go about things.

I thought it was just poorly worded. It can't be the most important thing for measuring time, yet have a characteristic that doesn't apply to a clock, because a clock is the most important thing for measuring time. Maybe it's a watch?

Let's apply the same reasoning to "most" then. Maybe it's a computer. You can only change it if you have the username and password and it can also measure time. Whoop-de-doo.

But it isn't nearly the most important tool for measuring time.

Time server (NTP servers). Pretty much the only thing they do is measure / tell time.

1. You didn't say time servers, you said computers. That's like me answering a riddle with the answer of computers, with 'electronics' -- be more specific.
2. It didn't say "my most important purpose", it said "I am the most important". The priority of what it does is irrelevant.