Variable scope within Angular factory promises

Here is the best example I have of how to implement this. I assumed some stuff about your controller and factory. Note that you need to inject $q into your factory to use deferreds. This is how I do it in my app.

http://jsfiddle.net/JimTheDev/2zLEp/

/*
  NOTE: make sure to inject $q into your factory before
  this. For the purposes of my example, let's assume your
  factory looks like this:
*/
app.factory('mySweetFactory', ['$q', function($q) {
    return {

        // You probably have a bunch of other methods like
        // this one in your factory.
        myothermethod: function(argument1, argument2) { 
            /* some code here */
        },

        update: function (staffList) {

            // Create a new deferred object
            var deferred = $q.defer();

            // get local last updated timestamp
            var lastUpdatedLocal = "";
            for (i=0; i < staffList.length; i++) {
                // get largest (most recent) timestamp
                if(staffList[i].entry_date > lastUpdatedLocal) {
                    lastUpdatedLocal = staffList[i].entry_date;
                }
            }

            // Make a call to the remote api
            $http.get('REMOTE_API_CALL').then(function(response) {

                // Get the remote last updated
                // time from the response
                var lastUpdatedRemote = response.data[0].entry_date;

                // Check if local or remote is larger
                // You probably need to modify this 
                // depending on your datetime formatting. 
                // I am assuming an epoch string.
                if(lastUpdatedRemote > lastUpdatedLocal)
                {
                    // Resolve the deferred and return
                    // the data so that the controller can
                    // use it to update the view
                    deferred.resolve(response.data);
                } else {
                    // Since we don't need to do anything,
                    // reject the deferred
                    deferred.reject();
            });

            // Return the promise from the deferred object
            // immediately. We will eventually either 
            // resolve or reject it, but at the start, it
            // will be in a pending state.
            return deferred.promise;
        }
    };
}]);

app.controller('mySweetController', ['mySweetFactory', function(mySweetFactory){
    $scope.doUpdate = function() {
        mySweetFactory.update($scope.staffList).then(function(newStaffList) {
            // Promise was resolved. List needs updating!
            $scope.staffList = newStaffList;
        }, function(){
            // Promise was rejected. No update needed!
        });
    };
}]);