'use strict';
(function () {
    var byallApp = angular.module('byallApp');
    byallApp.controller('ActivitieController', ['$scope', '$log', 'httpGetService',       '$rootScope', 'httpPostService', '$moment',
        function ($scope, $log, httpGetService, $rootScope, httpPostService, $moment) {

        $scope.activities = [];//array that holds the objects to be displayed in table.
        ....//a lot of normal code here. No code at all that updates the $scope.activities array is ever called outside some other function.

        //function that calculates the effort, uses momentjs
        this.calculateEffortFromValues = function (finalDate, initialDate) {
            $log.info('Executing calculateEffortFromValues');
            var initial = $moment(new Date(initialDate));
            var final = $moment(new Date(finalDate));
            var duration = $moment.utc(final.diff(initial)).format("HH:mm");
            $log.info('duration: ' + duration);
            return (duration);
        }

    }]);
})();

<div ng-controller="ActivitiesController as activitiesCtrl">
....
<tbody>
        <tr ng-repeat="activityList in activities">
            <td>{{activityList.initialDate | date : 'dd/MM/yyyy'}}</td>
            <td>{{activityList.initialDate| date : 'hh:mm a'}}</td>
            <td>{{activityList.endDate | date : 'hh:mm a'}}</td>
            **<td>{{activitiesCtrl.calculateEffortFromValues(activityList.endTime, activityList.initialTime)}}</td>**
            <td>{{activityList.codContract}}</td>
            <td>{{activityList.description}}</td>
            <td>
                <button class="btn btn-danger btn-mini" ng-click="deleteRow(row)" ng-hide="isTemp($index)"><img
                        width="25px" height="25px" title="Delete Activity!" src="img/trash.ico"/></button>
            </td>
        </tr>
        </tbody>