It's September 1, 2007 and this error is still occurring for me.  I downloaded and reinstalled both the ASP.NET AJAX extensions and the toolkit this week with the same results so I'm not sure it has been fixed.

Hi

I too updated AjaxControlToolkit but it doesn't work for me. It still displaying same error Sys.ArgumentUndefinedException. My date format is dd-MMM-yyyy.

Have u got any solution?

Not sure if this will work in all situations, but I had the error and fixed it this way:

In the page that uses the Date.parseLocale funciton (or the CalendarExtender, or whatever), in the Page_PreRender call, add a startup script (Page.ClientScript.RegisterStartupScript(Type type, string key, string script, bool addScriptTags) with the following as "script" and true as addScriptTags:

Sys.CultureInfo.prototype._getAbbrMonthIndex = function(value) {
    if (!this._upperAbbrMonths) {
        this._upperAbbrMonths = this._toUpperArray(this.dateTimeFormat.AbbreviatedMonthNames);
    }
    return Array.indexOf(this._upperAbbrMonths, this._toUpper(value));
};

Again, this works in my case, your mileage may vary...