fix(dialog): fix incorrect start time calculation logic

1. 修复使用未定义的endDate变量的bug
2. 添加日志打印endTime方便调试
3. 改用从服务器获取的endTime计算起始时间

src/components/dialog/addClassDialog.vue

@@ -186,10 +186,11 @@
           // 获取endTime为现在
           // let endDate = new Date();
           let endTime = await this.getServerTime()
-
+          console.log('endTime',endTime);
+          
           // 随机20~50分钟
           let randomMinutes = Math.floor(Math.random() * 31) + 20;
-          let startDate = new Date(endDate.getTime() - randomMinutes * 60 * 1000);
+          let startDate = new Date(new Date(endTime).getTime() - randomMinutes * 60 * 1000);
           let startTime = startDate.toLocaleString("zh-CN", {
             hour12: false,
             timeZone: "Asia/Shanghai"