String "ISO8601 date string" to PHP

Good morning. I'm working with FullCalendar. When I create an event, it returns a string "ISO8601 date string", I would like to translate it to PHP (date ("Y-m-d H: i: s"))

I know that to convert to an ISO8601 cademna is date ("c", strotime ($ date)), but let's say that what I want to do is the other way round.

The problem is when I do the following:

$cadenaenISO8601 = "1508112000000";
$date = date("Y-m-d H:i:s", strotime($cadena);

or

$date = date("Y-m-d H:i:s", intval($cadena);

It returns me wrong values.

Edit: $ task if it returns, anyway it has nothing to do and should not have done, since it only generates confusion. The issue is that when I click on a day to create an event, it returns start and end, with the string I put below

 $('#calendar').fullCalendar({
    // put your options and callbacks here
    locale: '<?php echo $locale ?>',
    header: {
        left: 'prev,next today',
        center: 'title',
        right: 'month,agendaWeek,agendaDay'
    },
    defaultView: 'month',
    navLinks: true, // can click day/week names to navigate views
    selectable: true,
    selectHelper: true,
    select: function(start, end) {

        window.location="{{ url('task_create') }}"+"/"+start+"/"+end;
    },

    editable: true,
    height:650,
    events : [


    ]
})

});

With this string:

Start: string(13) "1508198400000"

End: string(13) "1508284800000"