fix but

app/controllers/exercises_controller.rb

@@ -1016,10 +1016,15 @@ class ExercisesController < ApplicationController
             @exercise_user_current.update_attribute("start_at",Time.now)
           end
         end
-        if @exercise.time > 0
+        ex_time = @exercise.time
+        if ex_time > 0
+          time_mill = ex_time * 60   #转为秒
+          exercise_end_time = @exercise.end_time.present? ? @exercise.end_time.to_i : 0
           exercise_user_start = @exercise_user_current.present? ? @exercise_user_current.start_at.to_i : 0
-          exercise_user_left_time = Time.now.to_i - exercise_user_start
-          time_mill = @exercise.time * 60   #转为毫秒
+          if (exercise_user_start + time_mill) > exercise_end_time
+            time_mill = exercise_end_time - exercise_user_start   #如果开始答题时间加试卷的限时长大于试卷的截止时间，则以试卷的截止时间到开始答题时间为试卷的限时
+          end
+          exercise_user_left_time = Time.now.to_i - exercise_user_start   #用户已回答的时间
           @user_left_time = (time_mill < exercise_user_left_time) ? nil : (time_mill - exercise_user_left_time)   #当前用户对试卷的回答剩余时间
         end