Separate an instant from its local display
JavaScript Date represents an instant as milliseconds relative to an epoch. A time zone controls how that instant is shown as a calendar date and clock time.
World Clock formats the same instant for each selected zone. Changing from Seoul to London changes the local representation, not the moment being compared.
More digits do not prove more accuracy
The optional milliseconds are digits from the device clock. The browser cannot paint a new frame for every millisecond, and scheduling delays can postpone the visible update.
For a simple check, compare Clock Garden with the device's own clock. Agreement shows they share a source; it does not show that either is synchronized to an external reference.
What to check when the time looks wrong
Check the operating system's date, time zone, and automatic time settings. Then reload the page if you changed the time zone. Clock Garden cannot correct the operating system clock itself.
A remote website may use its own clock and request-processing rules. This tool cannot tell you exactly when that server will accept a registration or ticket request.
Where leap seconds fit
JavaScript Date timestamps do not count leap seconds as extra seconds. This display is not designed to show an inserted 23:59:60 second or to compare UTC with an atomic timescale.
Use an authoritative time service for a traceable reference and purpose-built equipment for measurement that requires calibration. This browser display is for everyday time awareness.