Date.getTimezoneOffset ignores daylight saving time on Windows #112

On Windows getTimezoneOffset computes mktime(localtime) - mktime(gmtime), but gmtime sets tm_isdst = 0, so the UTC fields are read as standard time and the offset never includes DST: in New York in July it returns 300 where Linux and the other targets return 240. The Windows-specific hx_windows_offset, already used for negative times, gives the right value.