You will need to convert the value if you are passing dates back and forth. In javascript,
var d = new Date()
d.setTime(-62135575200000);
alert(d.toDateString());
See the question Converting .NET DateTime to JSON and related answers there.
The following shows the two ways commented upon to move the dates.
in my code behind:
 [WebMethod]
 public static DateTime loadDate()
 {
     return DateTime.Now;
 }
 [WebMethod]
 public static double loadDateTicks()
 {
     return DateTime.Now.UnixTicks();
 }
 public static class ExtensionMethods
 {
    // returns the number of milliseconds since Jan 1, 1970 (useful for converting C# dates to JS dates)
    public static double UnixTicks(this DateTime dt)
    {
        DateTime d1 = new DateTime(1970, 1, 1);
        DateTime d2 = dt.ToUniversalTime();
        TimeSpan ts = new TimeSpan(d2.Ticks - d1.Ticks);
        return ts.TotalMilliseconds;
    }
 }
All credit for that extension method goes to "Jeff Meatball Yang".
My front end test is the following:
function LoadDates() {
        $.ajax({
            url: "Default.aspx/loadDate",
            type: "POST",
            data: "{}",
            contentType: "application/json; charset=utf-8",
            success: function (msg) {
                var re = /-?\d+/;
                var d = new Date(parseInt(re.exec(msg.d)[0]));
                alert(d.toDateString());
            },
            dataType: "json"
        });
        $.ajax({
            url: "Default.aspx/loadDateTicks",
            type: "POST",
            data: "{}",
            contentType: "application/json; charset=utf-8",
            success: function (msg) {
                var dt = new Date(msg.d);
                alert(dt.toDateString());
            },
            dataType: "json"
        });
    }