The number of microseconds since the "Unix epoch" 1970-01-01T00:00:00Z (UTC).
This value is independent of the time zone.
This value is at most
8,640,000,000,000,000,000us (100,000,000 days) from the Unix epoch.
In other words: microsecondsSinceEpoch.abs() <= 8640000000000000000.
Note that this value does not fit into 53 bits (the size of a IEEE double). A JavaScript number is not able to hold this value.