HTML表单数据提交

怎么才能点击submit后先验证input内容，符合条件提交表单，不符合则回复文本呢

<form  method="post"onsubmit="return checkip()">
            <label class='lab' for="exampleInputName2">start_time</label>
            <input type="datetime-local" class="form-control startTimewater" id="exampleInputName2" name="start_time">

            <label  class='lab' for="exampleInputEmail2">end_time</label>
            <input type="datetime-local" class="form-control endTimewater" id="exampleInputEmail2" name="end_time">
            <button type="submit" class="btn btn-info btn-search" id="submit">提交</button> 
    </form>

<script src="jquery-1.12.4.min.js"></script>
<script type="text/javascript">
 function checkip() {
        var start_time_water = $('#exampleInputName2').val()
        var end_time_water = $('#exampleInputEmail2').val()
        if (start_time_water=="" || end_time_water==""){
            alert('请输入正确的格式，时间不能为空！')
            return false;
        }
        else{
            return true;   
        }
    }

</script>

参考这个 。