修复一个bug：完成列表界面的用时显示不正确

lib/logic/done_task_page_logic.dart

@@ -45,6 +45,8 @@ class DoneTaskPageLogic {
     Duration diff = timeStart.difference(timeEnd);
     final context = _model.context;
 
+    debugPrint("差别:${timeStart}   end:${timeEnd}");
+
     return diff.inDays == 0
         ? "${DemoLocalizations.of(context).hours(diff.inHours)}"
         : "${DemoLocalizations.of(context).days(diff.inDays)}";

lib/logic/edit_page_task_logic.dart

@@ -259,7 +259,7 @@ class EditTaskPageLogic {
       account: account,
       taskType: _model.taskIcon.taskName,
       taskDetailNum: _model.taskDetails.length,
-      createDate: DateTime.now().toIso8601String(),
+      createDate: _model?.createDate?.toIso8601String() ?? DateTime.now().toIso8601String(),
       startDate: _model.startDate?.toIso8601String(),
       deadLine: _model.deadLine?.toIso8601String(),
       detailList: _model.taskDetails,
@@ -283,6 +283,7 @@ class EditTaskPageLogic {
       _model.deadLine = DateTime.parse(oldTaskBean.deadLine);
       if(oldTaskBean.startDate != null)
       _model.startDate = DateTime.parse(oldTaskBean.startDate);
+      _model.createDate = DateTime.parse(oldTaskBean.createDate);
       _model.changeTimes = oldTaskBean.changeTimes ?? 0;
       _model.taskIcon = oldTaskBean.taskIconBean;
       _model.currentTaskName = oldTaskBean.taskName;

lib/model/edit_task_page_model.dart

@@ -23,6 +23,10 @@ class EditTaskPageModel extends ChangeNotifier{
   DateTime deadLine;
   //开始日期
   DateTime startDate;
+  
+  //创建日期
+  DateTime createDate;
+  
   TaskIconBean taskIcon;
   String currentTaskName = "";
   int changeTimes = 0;