diff --git a/app/models/user_stat.rb b/app/models/user_stat.rb
index 65f48d62dc3..c4806af20b7 100644
--- a/app/models/user_stat.rb
+++ b/app/models/user_stat.rb
@@ -101,7 +101,7 @@ class UserStat < ActiveRecord::Base
   end
 
   def self.cache_last_seen(id, val)
-    $redis.set(last_seen_key(id), val)
+    $redis.setex(last_seen_key(id), MAX_TIME_READ_DIFF, val)
   end
 
   protected
diff --git a/spec/models/user_stat_spec.rb b/spec/models/user_stat_spec.rb
index 43e73819a14..be844c015d1 100644
--- a/spec/models/user_stat_spec.rb
+++ b/spec/models/user_stat_spec.rb
@@ -76,6 +76,15 @@ describe UserStat do
     let(:user) { Fabricate(:user) }
     let(:stat) { user.user_stat }
 
+    it 'always expires redis key' do
+      # this tests implementation which is not 100% ideal
+      # that said, redis key leaks are not good
+      stat.update_time_read!
+      ttl = $redis.ttl(UserStat.last_seen_key(user.id))
+      expect(ttl).to be > 0
+      expect(ttl).to be <= UserStat::MAX_TIME_READ_DIFF
+    end
+
     it 'makes no changes if nothing is cached' do
       $redis.del(UserStat.last_seen_key(user.id))
       stat.update_time_read!