I've used this:
//UTC() returns milliseconds, mktime returns seconds
document.write(Date.UTC(1979, 8, 0, 23) / 1000);
echo mktime(0, 0, 0, 9, 1, 1979);
and got the same output from both.
Seems like Date()'s month and day values are 0-based, and that my server has wrong time zone settings.
Oh, and use 9 instead of 09, or PHP tries to treat the number as octal (and fail).