为什么angular $ http将datetime字符串转换为unix时间戳？

I have a PHP file which is retrieving value, id and time. Below is an example of the output of the PHP file when accessed directly:

//Using this 
$print($json_encode($data))

//Returns this
"data":[  
  {  
    "y":7.76,
    "x":"2015-05-04 01:23:37",
    "CoilId":"E5E001B11"
  }
]

Now when I use my factory method "getData" to retrieve the data and console log it, I end up with this:

"data":[  
  {  
    "y":7.76,
    "x":1444017905000,
    "CoilId":"E5E001B11"
  }
]

Here is my factory:

myApp.factory('Widget', ['$http', function($http) {

var promise;

return{

    getData: function(widgetType, widgetId, machineId, start, end){
        promise = $http({
          url: "angular/data/testData.php",
          method: "POST",
          headers: {'Content-Type': 'application/x-www-form-urlencoded'},
          data:  $.param({widgetType: widgetType, widgetId: widgetId, 
                         machineId: machineId, start: start, end: end })
        })
        .success(function(data, status, headers, config) {
          console.log(data);
        })
        .error(function(data, status, headers, config) {
          console.log("Data: " + data + "
" + "Status" + status + "
"+ "Headers"
                       + headers + "
" + "Config" + config + "
");
        });

        return promise;
    },

};

}]);