为什么jsp页面用表单提交后时分秒变成0？

Question

<input type="text" class="input" id="start" style="width: 200px;" name="start" readonly="readonly" value="<fmt:formatDate value='${activity.start}' pattern='yyyy-MM-dd HH:mm:ss'/>" validate="required" data-requiredmsg="请选择开始时间" onfocus="WdatePicker({maxDate:'#F{$dp.$D(\'end\')}',dateFmt:'yyyy-MM-dd HH:mm:ss'})"/>