diff --git a/app/models/topic.rb b/app/models/topic.rb
index 7cf68865200..8e761c33f03 100644
--- a/app/models/topic.rb
+++ b/app/models/topic.rb
@@ -1339,10 +1339,9 @@ class Topic < ActiveRecord::Base
         topic_timer.execute_at = num_hours.hours.from_now if num_hours > 0
       else
         timestamp = utc.parse(time)
-        raise Discourse::InvalidParameters unless timestamp
+        raise Discourse::InvalidParameters unless timestamp && timestamp > utc.now
         # a timestamp in client's time zone, like "2015-5-27 12:00"
         topic_timer.execute_at = timestamp
-        topic_timer.errors.add(:execute_at, :invalid) if timestamp < utc.now
       end
     end
 
diff --git a/spec/models/topic_spec.rb b/spec/models/topic_spec.rb
index 582db5785ac..8ccac8ac3db 100644
--- a/spec/models/topic_spec.rb
+++ b/spec/models/topic_spec.rb
@@ -1756,10 +1756,13 @@ describe Topic do
 
     it "sets a validation error when given a timestamp in the past" do
       freeze_time now
-      topic.set_or_create_timer(TopicTimer.types[:close], '2013-11-19 5:00', by_user: admin)
 
-      expect(topic.topic_timers.first.execute_at).to eq_time(Time.zone.local(2013, 11, 19, 5, 0))
-      expect(topic.topic_timers.first.errors[:execute_at]).to be_present
+      expect do
+        topic.set_or_create_timer(
+          TopicTimer.types[:close],
+          '2013-11-19 5:00', by_user: admin
+        )
+      end.to raise_error(Discourse::InvalidParameters)
     end
 
     it "sets a validation error when give a timestamp of an invalid format" do
diff --git a/spec/requests/api/topics_spec.rb b/spec/requests/api/topics_spec.rb
index 22a0b64b5ec..a2592cae036 100644
--- a/spec/requests/api/topics_spec.rb
+++ b/spec/requests/api/topics_spec.rb
@@ -954,7 +954,7 @@ describe 'topics' do
           category_id: { type: :string, nullable: true },
         }
 
-        let(:request_body) { { time: '2020-07-11+18:00-06:00', status_type: 'close' } }
+        let(:request_body) { { time: Time.current + 1.day, status_type: 'close' } }
         let!(:topic_post) { Fabricate(:post) }
         let(:id) { topic_post.topic.id }
 
diff --git a/spec/requests/topics_controller_spec.rb b/spec/requests/topics_controller_spec.rb
index 214263be022..92d1eed19dd 100644
--- a/spec/requests/topics_controller_spec.rb
+++ b/spec/requests/topics_controller_spec.rb
@@ -2932,6 +2932,19 @@ RSpec.describe TopicsController do
       end
     end
 
+    context 'when time is in the past' do
+      it 'returns an error' do
+        freeze_time
+        sign_in(admin)
+
+        post "/t/#{topic.id}/timer.json", params: {
+          time: Time.current - 1.day,
+          status_type: TopicTimer.types[1]
+        }
+        expect(response.status).to eq(400)
+      end
+    end
+
     context 'when logged in as an admin' do
       before do
         freeze_time