The DOMHighResTimeStamp type is a double and is used to store a time value in milliseconds.
This type can be used to describe a discrete point in time or a time interval (the difference in time between two discrete points in time). The starting time can be either a specific time determined by the script for a site or app, or the time origin.
The time, given in milliseconds, should be accurate to 5 µs (microseconds), with the fractional part of the number indicating fractions of a millisecond. However, if the browser is unable to provide a time value accurate to 5 µs (due, for example, to hardware or software constraints), the browser can represent the value as a time in milliseconds accurate to a millisecond. Also note the section below on reduced time precision controlled by browser preferences to avoid timing attacks and fingerprinting.
Further, if the device or operating system the user agent is running on doesn't have a clock accurate to the microsecond level, they may only be accurate to the millisecond.