diff --git a/app/controllers/homework_common_controller.rb b/app/controllers/homework_common_controller.rb
index 031a228ce..cb6c1505b 100644
--- a/app/controllers/homework_common_controller.rb
+++ b/app/controllers/homework_common_controller.rb
@@ -105,8 +105,8 @@ class HomeworkCommonController < ApplicationController
         else
           @homework.publish_time = params[:homework_common][:publish_time]
         end
-        param_end_time = Time.parse(params[:homework_common][:end_time])
-        homework_end_time = @homework.end_time ? @homework.end_time : ''
+        param_end_time = Time.parse(params[:homework_common][:end_time]).to_s
+        homework_end_time = @homework.end_time ? @homework.end_time.to_s : ''
         if homework_end_time != param_end_time
           if homework_end_time > param_end_time
             @homework.student_works.where("work_status = 1").each do |st|