I have a function which works well, for converting dates from a webservice returned in json format. The webservices gives dates in the following type of format:
You could also try moment.js. A 6.5kb library for formatting dates
var m = moment( new Date() );
m.format( "DD/MM/YYYY HH:mm");
                                                                        If you don't use a library, then you have to do some work, that is you have to put the "0" yourself.
Instead of simply concatenating day, you need to concatenate
(day<10 ? '0'+day : day)
and the same for the other fields.
But note that there are good javascript libraries filling this kind of gap. I personally used datejs for date manipulations.
I'd suggest using a library for this kind of thing -- something like Moment.js would do the job perfectly (and give you a load more functionality like date addition/subtraction into the bargain).
With moment.js, your code could look like this:
function HumanDate(date) {
    return moment(date).format('MM/DD/YYYY HH:mm');
}
usage example:
alert(HumanDate("\/Date(1373875200000)\/"));
//alerts "07/15/2013 09:00"
Hope that helps.