diff --git a/README.md b/README.md
index 3b3a7d0..d941bb1 100644
--- a/README.md
+++ b/README.md
@@ -111,6 +111,7 @@ $subscription = $subscriptions->create([
 ]);
 
 $subscription->checkout();             // bill the first period
+$subscription->checkout(startsAt: $domain->registered_at); // already running: bill the period that started then
 $subscription->pause();                // expires when the paid period ends
 $subscription->resume();               // expired subscriptions start a new period
 $subscription->revoke(refund: false);  // cancel
diff --git a/README.stub b/README.stub
index a1f5d9d..dc76d3c 100644
--- a/README.stub
+++ b/README.stub
@@ -111,6 +111,7 @@ $subscription = $subscriptions->create([
 ]);
 
 $subscription->checkout();             // bill the first period
+$subscription->checkout(startsAt: $domain->registered_at); // already running: bill the period that started then
 $subscription->pause();                // expires when the paid period ends
 $subscription->resume();               // expired subscriptions start a new period
 $subscription->revoke(refund: false);  // cancel
diff --git a/src/Id/Resources/AppSubscription.php b/src/Id/Resources/AppSubscription.php
index 3ad1974..fb1c32d 100644
--- a/src/Id/Resources/AppSubscription.php
+++ b/src/Id/Resources/AppSubscription.php
@@ -4,6 +4,7 @@ namespace Anikeen\Id\Resources;
 
 use Anikeen\Id\Exceptions\ApiException;
 use Anikeen\Id\Support\AppRequest;
+use DateTimeInterface;
 use JsonSerializable;
 
 /**
@@ -58,11 +59,18 @@ class AppSubscription implements JsonSerializable
     /**
      * Activate the subscription and bill its first period.
      *
-     * @throws ApiException 409 "already_active", "billing_address_missing", …
+     * $startsAt (in the past) books a service that is already running, e.g. a domain registered
+     * earlier: the first period runs from $startsAt, is billed now and renews when it ends.
+     *
+     * @throws ApiException 409 "already_active", "billing_address_missing", "period_already_ended", …
      */
-    public function checkout(): self
+    public function checkout(DateTimeInterface|string|null $startsAt = null): self
     {
-        return $this->send('PUT', '/checkout');
+        if ($startsAt instanceof DateTimeInterface) {
+            $startsAt = $startsAt->format(DateTimeInterface::ATOM);
+        }
+
+        return $this->send('PUT', '/checkout', $startsAt === null ? null : ['starts_at' => $startsAt]);
     }
 
     /**
diff --git a/tests/Id/AppSubscriptionsTest.php b/tests/Id/AppSubscriptionsTest.php
index d094fe5..53e41ef 100644
--- a/tests/Id/AppSubscriptionsTest.php
+++ b/tests/Id/AppSubscriptionsTest.php
@@ -76,6 +76,17 @@ class AppSubscriptionsTest extends TestCase
         $this->assertSame(['refund' => true], $this->sentJson(4));
     }
 
+    public function test_a_running_service_is_checked_out_from_its_start(): void
+    {
+        $this->respond(201, $this->subscription());
+        $this->respond(200, $this->subscription(['active' => true]));
+
+        $subscription = $this->owner()->appSubscriptions()->create(['name' => '.de TLD', 'unit' => 'year', 'price' => '5.19']);
+        $subscription->checkout(new \DateTimeImmutable('2026-05-19 11:33:58', new \DateTimeZone('UTC')));
+
+        $this->assertSame(['starts_at' => '2026-05-19T11:33:58+00:00'], $this->sentJson(1));
+    }
+
     public function test_errors_carry_status_and_error_code(): void
     {
         $this->respond(409, ['message' => 'Product subscription has already been cancelled.', 'error' => 'already_cancelled']);