Validate date as user types it in

I have a form written in PHP. And I am trying to validate that the user inputs an end date that is after the start date, but I want an alert to pop up as soon as the user finished typing in the end date telling the user if they entered an invalid end date. I have seen people show how to validate and that's not an issue, I just want the alert to pop up essentially as they start typing.

This is the code for this segment of the form.

                    <div class="form-group col-md-3">
                        <label for="inputStartDate">Start Date</label>
                        <input type="date" class="form-control" name="startDate" id="inputStartDate" value="" placeholder="12/02/2018" required>
                    </div>
                    <div class="form-group col-md-3">
                        <label for="inputEndDate">End Date</label>
                        <input type="date" class="form-control" name="endDate" id="inputEndDate" value="" placeholder="" required>
                    </div>