Auto-close time can be entered in 3 ways, so a topic can close at any time

app/models/topic.rb

@@ -146,7 +146,7 @@ class Topic < ActiveRecord::Base
     self.bumped_at ||= Time.now
     self.last_post_user_id ||= user_id
     if !@ignore_category_auto_close and self.category and self.category.auto_close_days and self.auto_close_at.nil?
-      set_auto_close(self.category.auto_close_days)
+      set_auto_close(self.category.auto_close_days * 24)
     end
   end
 
@@ -602,9 +602,10 @@ class Topic < ActiveRecord::Base
     end
   end
 
+  # TODO: change this method, along with category's auto_close_days. Use hours.
   def auto_close_days=(num_days)
     @ignore_category_auto_close = true
-    set_auto_close(num_days)
+    set_auto_close(num_days * 24)
   end
 
   def self.auto_close
@@ -622,10 +623,27 @@ class Topic < ActiveRecord::Base
     end
   end
 
-  def set_auto_close(num_days, by_user=nil)
-    num_days = num_days.to_i
-    self.auto_close_at = (num_days > 0 ? num_days.days.from_now : nil)
-    if num_days > 0
+  # Valid arguments for the auto close time:
+  #  * An integer, which is the number of hours from now to close the topic.
+  #  * A time, like "12:00", which is the time at which the topic will close in the current day
+  #    or the next day if that time has already passed today.
+  #  * A timestamp, like "2013-11-25 13:00", when the topic should close.
+  #  * A timestamp with timezone in JSON format. (e.g., "2013-11-26T21:00:00.000Z")
+  #  * nil, to prevent the topic from automatically closing.
+  def set_auto_close(arg, by_user=nil)
+    if arg.is_a?(String) and matches = /^([\d]{1,2}):([\d]{1,2})$/.match(arg.strip)
+      now = Time.zone.now
+      self.auto_close_at = Time.zone.local(now.year, now.month, now.day, matches[1].to_i, matches[2].to_i)
+      self.auto_close_at += 1.day if self.auto_close_at < now
+    elsif arg.is_a?(String) and arg.include?('-') and timestamp = Time.zone.parse(arg)
+      self.auto_close_at = timestamp
+      self.errors.add(:auto_close_at, :invalid) if timestamp < Time.zone.now
+    else
+      num_hours = arg.to_i
+      self.auto_close_at = (num_hours > 0 ? num_hours.hours.from_now : nil)
+    end
+
+    unless self.auto_close_at.nil?
       self.auto_close_started_at ||= Time.zone.now
       if by_user and by_user.staff?
         self.auto_close_user = by_user