修复 数据库删除以前数据时，月份减法出错。 导致12月 肯定会错！！！

Project.SQLiteHelper/SQLiteHelper/SQLiteDbContext.cs

@@ -331,9 +331,7 @@ namespace SQLite
                 return;//no datas
 
             DateTime curr_month = new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1);
-            int curr_total_month = curr_month.Year * 12 + curr_month.Month;
-            int limit_total_month = curr_total_month - month;
-            DateTime limit_month = new DateTime( limit_total_month/12, limit_total_month % 12, 1);
+            DateTime limit_month = curr_month.AddMonths(-month);
             DateTime start_month = firstTimes.Min();
             start_month = new DateTime(start_month.Year, start_month.Month, 1);
             if (start_month < limit_month)