XML descendants internal method called on incompatible Date

I just got this errormessage in Firebug's console. What the F? What’s wrong with this code?

var d=new Date();
console.info("offset: %s, utc hours: %s", d..getTimezoneOffset(), d.getUTCHours());

Oops. Notice the two dots before getTimezoneOffset.

That triggers an E4X error, which is apparently switched on in the Firebug console…