The link from Robert is good, but we should strive to answer the question here, not to just post links.
Here’s a quick function that does what you need. http://jsfiddle.net/Aaa6r/
function deserializeDotNetDate(dateStr) {
var matches = /\/Date\((\d*)\)\//.exec(dateStr);
if(!matches) {
return null;
}
return new Date( parseInt( matches[1] ) );
}
deserializeDotNetDate("/Date(1304146800000)/");