Credit: Official US Time, NIST and USNO.
The Autumnal Equinox (is it the Vernal Equinox for those of you in the southern hemisphere?) occurs at 4:03 on September 22, 2006.
But ... the equinox occurs when the sun "crosses" the equator (for you geocentrists), or the equator passes "beneath" the sun (for you heliocentrists), or something. It's the same moment in time for everyone on the planet, even if the sun's not visible to them. It can't be "4:03" for everyone.
So what time is 4:03 anyway? Times of equinoxes are given in Universal Time, abbreviated as UT or UTC (more politically correct than Greenwich Mean Time, and close enough for most of us). To know when the equinox occurs for you, you need to convert from UTC to your local time.
I'm still on Eastern Daylight Savings Time, or EDT, which is four hours behind UTC. So the equinox occurs for me just after midnight tonight, the time I've given this post. When we "fall back" the clocks, I'll be back on Eastern Standard Time, EDT, which is five hours behind UTC.
LinksU.S. Naval Observatory: Earth's Seasons and Time Service Department