currently cannot read value from template attribute which is a controller define model, so change the defaultParams if you need a different init option, or you can provider a constant value to template attribute.

myController.js

angular.module('myApp').controller('myController', function($scope) {
  $scope.daterangeModel = {
    start: moment().subtract('days', 5),
    end: moment()
  };
});

ng-daterangepicker.js

'use strict';

angular.module('ngDaterangepicker', [])
  .constant('defaultParams', {
    opens: 'right',
    ranges: {
      'Today': [moment(), moment()],
      'Yesterday': [moment().subtract('days', 1), moment().subtract('days', 1)],
      'Last 7 Days': [moment().subtract('days', 6), moment()],
      'Last 30 Days': [moment().subtract('days', 29), moment()],
      'This Month': [moment().startOf('month'), moment().endOf('month')],
      'Last Month': [moment().subtract('month', 1).startOf('month'), moment().subtract('month', 1).endOf('month')]
    },
    startDate: moment(),
    endDate: moment()
  })
  .directive('daterangepicker', function (defaultParams, $timeout) {
    return {
      restrict: 'A',
      scope: false,
      require: '?ngModel',
      link: function postLink(scope, element, attrs, ngModel) {
        if(!ngModel) {
          console.log('ngModel is unavailable')
          return;
        }
        var refElement = $(element);
        // Model -> UI
        ngModel.$render = function() {
          //TBD
        }
        var params = $.extend(defaultParams, {
          startDate: attrs.startDate,
          endDate: attrs.endDate,
          minDate: attrs.minDate,
          maxDate: attrs.maxDate,
          dateLimit: attrs.dateLimit,
          showDropdowns: attrs.showDropdowns,
          showWeekNumbers: attrs.showWeekNumbers,
          timePicker: attrs.timePicker,
          timePickerIncrement: attrs.timePickerIncrement,
          timerPicker12Hour: attrs.timePicker12Hour,
          ranges: attrs.ranges,
          opens: attrs.opens,
          buttonClasses: attrs.buttonClasses,
          applyClass: attrs.applyClaass,
          cancelClass: attrs.cancelClass,
          format: attrs.format,
          separator: attrs.separator,
          locale: attrs.locale
        });
        refElement.daterangepicker(params, function(start, end){
          console.log(start + '-' + end);
          ngModel.$setViewValue({start:start, end:end});
          scope.$digest();
        });
      }
    };
  });

template

<p ng-controller="myController">
  <button class="btn" ng-Model="daterangeModel" daterangepicker opens="left">
    <i class="icon-calendar icon-large"></i>
    <span>{{daterangeModel.start}} - {{daterangeModel.end}}</span>
    <b class="caret"></b>
  </button>
</p>