var d = new Date("garbage");
1. "NaN" (stands for Not a Number) in IE and
2. "Invalid Date" in Firefox
Also, if you're looking for a jquery plugin for a popup calendar, I'd recommend using Kevin Luck's date-picker (http://www.kelvinluck.com/assets/jquery/datePicker/v2/demo/). We used this at a client and it worked consistently across IE, FireFox and Chrome.