Media diagnostics · Primary-source explanation; examples are illustrative. Instructions are based on publisher guidance; a UK IPTV account was not tested on this platform.
Choose the clock for the question
MDN distinguishes Date.now, tied to the system clock and Unix epoch, from performance.now, which measures against a monotonic time origin. System or user clock adjustments can affect the former. When timing a short player operation, a wall clock correction can therefore distort the apparent elapsed duration. A date stamp is still useful for identifying when the test happened, but that is a different purpose from measuring a short interval.
Work through a clock jump
An original conceptual example begins a task at a displayed system time of 12:00:00. During the next three seconds the clock is corrected backwards by ten seconds. Subtracting the two wall time readings can produce a negative apparent interval despite the task moving forward. The example does not describe a measured browser defect. It shows why a duration report should identify the clock and retain the event date as a separate field.
Do not generalise beyond the timing scope
The documentation notes platform differences involving whether high resolution timing continues during operating system sleep, and privacy measures can coarsen precision. Do not promote one API into a perfect long duration stopwatch for every environment. For a short comparison, retain the same clock origin and avoid subtracting values from unrelated contexts. This is a measurement design article, not a reason to disable time synchronisation or a claimed timing result for UK IPTV.