Question

The Date constructor in JavaScript/ECMAScript/JScript allows passing the number of milliseconds since midnight, 1/1/1970. Nowhere 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?

Answer 1

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.