diff --git a/lib/client.rb b/lib/client.rb
index f2cbf96..4c97ae5 100644
--- a/lib/client.rb
+++ b/lib/client.rb
@@ -220,7 +220,7 @@ def insert_many(args)
       end
 
       if unique_opts.by_period
-        lower_period_bound = truncate_time(@time_now_utc.call, unique_opts.by_period).utc
+        lower_period_bound = truncate_time(insert_params.scheduled_at || @time_now_utc.call, unique_opts.by_period).utc
 
         unique_key += "&period=#{lower_period_bound.strftime("%FT%TZ")}"
       end
diff --git a/spec/client_spec.rb b/spec/client_spec.rb
index c818a8b..1f392a3 100644
--- a/spec/client_spec.rb
+++ b/spec/client_spec.rb
@@ -291,6 +291,24 @@ def check_bigint_bounds(int)
         expect(insert_res.unique_skipped_as_duplicated).to be false
       end
 
+      it "inserts a new unique job with period determined from `scheduled_at`" do
+        job_args = ComplexArgs.new(customer_id: 1, order_id: 2, trace_id: 3, email: "john@example.com")
+        insert_opts = River::InsertOpts.new(
+          scheduled_at: now + 3600,
+          unique_opts: River::UniqueOpts.new(
+            by_period: 15 * 60
+          )
+        )
+
+        insert_res = client.insert(job_args, insert_opts: insert_opts)
+        expect(insert_res.job).to_not be_nil
+        expect(insert_res.unique_skipped_as_duplicated).to be false
+
+        unique_key_str = "&kind=#{insert_res.job.kind}" \
+          "&period=#{client.send(:truncate_time, now + 3600, 15 * 60).utc.strftime("%FT%TZ")}"
+        expect(insert_res.job.unique_key).to eq(Digest::SHA256.digest(unique_key_str))
+      end
+
       it "skips unique check if unique opts empty" do
         job_args = SimpleArgsWithInsertOpts.new(job_num: 1)
         job_args.insert_opts = River::InsertOpts.new(