UTC Behavior

TODO(65784): Populate this page to document the behavior that users of the UTC clock can expect, including the following points as a minimum:

• How to get a clock handle.
• Clock handle is a process variable, if the parent didn’t supply it it won’t be available.
• Under no circumstances will the clock ever be earlier than backstop.
• Until UTC is available the clock will remain fixed at the backstop time.
• ZX_CLOCK_STARTED may be waited on to learn when time is available.
• The clock may step either forward or backward and there is no upper limit on the size of this step.
• Steps should be rare on most devices, except for the initial step from backstop time to the accurate time when the clock is first synchronized.
• The clock may run at a different rate than monotonic to compensate for oscillator errors or to gradually remove small errors, should never exceed a few hundred ppm in either direction.
• How to get clock details.
• Definition of error bound in clock details (when we agree this).
• Accuracy will vary from product to product but should be accurate to within a few hundreds of milliseconds on most devices most of the time.
• What secure time means (when we agree this).
• How to tell when time is secure (when we agree this).