A clock - in the general sense - is a way of measuring time. That could be a specific point in time or a duration. In fact, a specific point in time is just a duration
from a set point in time; this can be observed with the computer's internal clock, for one example. An "actual time" is also relative to location and subject to local time savings. Of course, this can be adjusted to a locale but the base for these times are the internal clock, which, you could say, is a stopwatch.
I'm not saying I disagree with you, by the way,
as you can probably see in my own timing library, I already have used the terms in the way you have suggested (Stopwatch, Timer, Duration)!
It's worth noting a few little things, though:
- Stopwatch should be just one word,
- a watch (or wristwatch) is just a clock for the wrist and
- stopwatches are also called stopclocks.