JavaScript date constructor and timezone

The Date constructor in JavaScript/ECMAScript/JScript allows passing the number of milliseconds since midnight, 1/1/1970. Nowhere 开发者_运维知识库can I find documentation whether this is midnight in the client machine's timezone, or midnight GMT. Which is it? Can it be relied on between different browsers and versions? Is this officially documented anywhere?

From the ECMAScript specification:

Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers from –9,007,199,254,740,991 to 9,007,199,254,740,991; this range suffices to measure times to millisecond precision for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.

The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.

The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0.

So to answer your question, it's Coordinated Universal Time.