No, I don’t think it’s terrible at all.
Time is a constant measure of potential change (a gram is always a gram), and the speed of time is the rate of that change.
The radioactive decay of Uranium will always take X seconds, but those seconds may occur at different rates than the same event when a given energetic context is applied to it, such as gravity or motion. One gravity A second may take longer to conclude than a gravity B second. This is because energy modifies the rate of change, without modifying it in any other way, so that all parts of the event occur in the same succession and thus take the same amount of time regardless of the speed of time.
I suppose quantic analysis would take a different approach. The question then would be what exact measurement you are referring to when you say “time,” how can you replicate the measurement, and how is the measurement affected when subjected to a given set of circumstances within a given, hermetic set of theoretical postulates, which themselves would require repeatable methods of measurement.
I would say yours is a pretty spot on definition of time within a context of relativistic physics.