angularjs validation update error status of another input field

Aloha. I have two fields for time input, start + end. I've created custom validations to check if field is blank, valid time, and start is before end time. Works except for the following:

• edit both times to be 7:45 and 7:50 respectively, no errors;
• edit end time to be before the start time, from 7:50 to 7:44, the validation does its job, i.e. $error.validAfter = true;
• ignore end time field and edit start time to be after the end time, from 7:45 to 7:47, ($error.validBefore error = true);
• edit start time to be before end time, from 7:47 to 7:43 - $error.validBefore = false;

At this point, $error.validAfter is still true for end time. Is there a way to update this after the validation for start time, fourth bullet, has been confirmed?

HTML:

<div data-ng-app="scheduleApp">
<h2>{{ "Schedule App" }}</h2>
<div class="time" data-ng-form="formtime">
    <div class="form-row">
        <span class="label">Start time:</span>
        <span class="field">
            <input type="text" name="startTime" ng-model="startTime" is-from placeholder="e.g. 7:50" required validatetime="{{ endTime }}" />
            <span class="error" ng-class="{required: formtime.startTime.$error.required, invalid: formtime.startTime.$error.validTime}">enter a valid time, <i>hh:mm</i></span>
            <span class="error" ng-show="formtime.startTime.$error.validBefore" ng-class="{invalid: formtime.startTime.$error.validBefore && !formtime.startTime.$error.validTime}">start time must be before the end time</span>
        </span>
        <span>{{ formtime.startTime.$error }}</span>
    </div>
    <div class="form-row">
        <span class="label">End time:</span>
        <span class="field">
            <input type="text" name="endTime" ng-model="endTime" placeholder="e.g. 7:55" required validatetime="{{ startTime }}" />
            <span class="error" ng-class="{required: formtime.endTime.$error.required, invalid: formtime.endTime.$error.validTime}">enter a valid time, <i>hh:mm</i></span>
            <span class="error" ng-show="formtime.endTime.$error.validAfter" ng-class="{invalid: formtime.endTime.$error.validAfter && !formtime.endTime.$error.validTime}">end time must be after the start time</span>
        </span>
        <span>{{ formtime.endTime.$error }}</span>
    </div>
</div>

Javascript:

var app = angular.module("scheduleApp", []);

function PMhour(hour) {
    var pmHours = [1,2,3,4,5,6];

    return pmHours.indexOf(hour) >= 0 ? (hour + 12) : hour;
}

app.directive("validatetime", function() {
    return {
        restrict:   "A"
        , require:  "ngModel"
        , link:
            function($scope, elem, attrs, ctrl) {
                var validTime       = false;
                var validCompare    = false;
                var isFrom          = attrs.isFrom == null ? false : true;
                var otherTime;

                attrs.$observe("validatetime", function(otherTimeVal) {
                    otherTime   = otherTimeVal;
                });

                ctrl.$parsers.unshift   (isValidTime, isValidTimeCompare);
                ctrl.$formatters.unshift(isValidTime, isValidTimeCompare);

                function isValidTime(value) {
                    var regex   = /^([0-9]|0[0-9]|1[0-9]|2[0-3]):[0-5][0-9]$/i;

                    validTime   = false;

                    if (value != null) {
                        if (value.length) {
                            validTime   = regex.test(value);
                        }
                    }

                    ctrl.$setValidity('validTime', validTime);

                    if (validTime) {
                        return value;
                    }
                }

                function isValidTimeCompare(value) {
                    var tmpFrom;
                    var tmpTo;

                    if (otherTime) {
                        validCompare    = (otherTime.length == 0);
                    }
                    else {
                        validCompare    = true;
                    }

                    if (validTime) {
                        if (otherTime.length) {
                            if (isFrom) {
                                tmpTo   = new Date(2000, 0, 1, PMhour(parseInt(otherTime.split(":")[0])), parseInt(otherTime.split(":")[1]));
                                tmpFrom = new Date(2000, 0, 1, PMhour(parseInt(value.split(":")[0]))    , parseInt(value.split(":")[1]));
                            }
                            else {
                                tmpFrom = new Date(2000, 0, 1, PMhour(parseInt(otherTime.split(":")[0])), parseInt(otherTime.split(":")[1]));
                                tmpTo   = new Date(2000, 0, 1, PMhour(parseInt(value.split(":")[0]))    , parseInt(value.split(":")[1]));
                            }

                            validCompare = (tmpFrom < tmpTo);
                        }
                    }

                    if (isFrom) {
                        ctrl.$setValidity("validBefore", validCompare); // from is before the other time
                    }
                    else {
                        ctrl.$setValidity("validAfter", validCompare);      // to is after the other time
                    }

                    return value;
                };
            }
    }
});

Plunker here: http://plnkr.co/edit/Innbv6lCYewefSgKhvGA?p=info
Sorry, I'm new to the plunker concept. Let me know if you cannot see the code. Mahalo for your assistance.