As wizzard pointed out, the correct method is,
new Date().getTime();
or under Javascript 1.5, just
Date.now();
From the documentation,
The value returned by the getTime method is the number of milliseconds since 1 January 1970 00:00:00 UTC.
If you wanted to make a time stamp without milliseconds you can use,
Math.floor(Date.now() / 1000);
I wanted to make this an answer so the correct method is more visible.
You can compare ExpExc's and Narendra Yadala's results to the method above at http://jsfiddle.net/JamesFM/bxEJd/, and verify with http://www.unixtimestamp.com/ or by running date +%s on a Unix terminal.