diff --git a/app/models/topic.rb b/app/models/topic.rb
index 55f016c4b44..a9169ad515a 100644
--- a/app/models/topic.rb
+++ b/app/models/topic.rb
@@ -1101,19 +1101,17 @@ SQL
       end
     else
       utc = Time.find_zone("UTC")
-      is_timestamp = time.is_a?(String)
-      now = utc.now
+      is_float = (Float(time) rescue nil)
 
-      if is_timestamp && time.include?("-") && timestamp = utc.parse(time)
+      if is_float
+        num_hours = time.to_f
+        topic_timer.execute_at = num_hours.hours.from_now if num_hours > 0
+      else
+        timestamp = utc.parse(time)
+        raise Discourse::InvalidParameters unless timestamp
         # 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 < now
-      else
-        num_hours = time.to_f
-
-        if num_hours > 0
-          topic_timer.execute_at = num_hours.hours.from_now
-        end
+        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 a1e60727b0b..a88441bb755 100644
--- a/spec/models/topic_spec.rb
+++ b/spec/models/topic_spec.rb
@@ -1392,6 +1392,18 @@ describe Topic do
       expect(topic.topic_timers.first.errors[:execute_at]).to be_present
     end
 
+    it "sets a validation error when give a timestamp of an invalid format" do
+      freeze_time now
+
+      expect do
+        topic.set_or_create_timer(
+          TopicTimer.types[:close],
+          '۲۰۱۸-۰۳-۲۶ ۱۸:۰۰+۰۸:۰۰',
+          by_user: admin
+        )
+      end.to raise_error(Discourse::InvalidParameters)
+    end
+
     it "can take a timestamp with timezone" do
       freeze_time now
       topic.set_or_create_timer(TopicTimer.types[:close], '2013-11-25T01:35:00-08:00', by_user: admin)