new Date().getTime() // returns same utc milliseconds for all system time zones
So, is new Date().getTime() returns utc milliseconds?
There is no such things as "UTC milliseconds". A millisecond is an SI unit that is one thousandth of a second.
ECMAScript Date instances hold a time value that is an offset in milliseconds from 1970-01-01T00:00:00Z (the ECMAScript epoch). As a consequence, Dates are considered to be UTC, but that's just a concept based on the underlying time value.