JQuery :: GetJSON Method - Scope Of Variables

I'm using jQuery's getJSON method to get the current date from a foreign server. I however, should this request fail (e.g. if the server is down), I want to have my page fall back to a sensible default.

What I'd like to do is this:
var date = "sensible default"
$.getJSON(url, function(data){ date = new Date(data.datetime); });
if request was successful date will contain new Date(data.datetime);
if request was unsuccessful date still contains "sensible_default"

What I have is this:
var timezone = "Europe/Berlin";
var date = "sensible_default";
$.getJSON("[URL]"+timezone+"&callback=?",
function(data){ date = new Date(data.datetime); });
alert(date);
But this produces an alert of "sensible_default", despite the fact that the getJSON request fired correctly.