angular js not validating datetimepicker

Question

I am using angular js with bootstrap datetimepicker and I have this input in my form:

    <input class="form-control" type="text" name="dob" id="datetimepicker" ng-model="user.dob" value="{{user.dob}}" ng-blur="blur4=true" required />
    <span class="error" ng-show="user.validate.step1.dob.$invalid">*</span>

and this is my jquery code for the datetimepicker:

$('#datetimepicker').datetimepicker({
        format: 'MM/DD/YYYY',
        viewMode: 'years'
    });

however when I click on the input and select a date from the datetimepicker, it says user.validate.step1.dob.$invalid, is invalid, why is that?

I have tried the following inside my directive:

link: function (scope) {
            scope.currentStepIndex = 0;
            scope.steps[scope.currentStepIndex].currentStep = true;
            $('#datetimepicker').datetimepicker({
                format: 'MM/DD/YYYY',
                viewMode: 'years',
            }).on("change.dp", function (selectedDate) {
                scope.$apply(function () {
                    ngModel.$setViewValue(dateText);
                });
            });
        }

and it still marks my dob as not valid.