diff --git a/src/Standing/StandingGrant.php b/src/Standing/StandingGrant.php
index 6eebc37..dab0942 100644
--- a/src/Standing/StandingGrant.php
+++ b/src/Standing/StandingGrant.php
@@ -290,7 +290,7 @@ public static function fromArray(array $data): self
         $expires = $data['expires_at'] ?? null;
         $expiresAt = \is_string($expires) ? date_create_immutable($expires) : null;
         if ($expires !== null && !$expiresAt instanceof \DateTimeImmutable) {
-            throw new \InvalidArgumentException("{$where} gives «expires_at» as a date and time, e.g. 2026-12-31T00:00:00Z");
+            throw new \InvalidArgumentException("{$where} gives «expires_at» as a date and time with its offset, in ISO 8601");
         }
 
         $supersedes = $data['supersedes'] ?? null;
diff --git a/tests/Standing/AStandingGrantNeverCrossesItsFloorsTest.php b/tests/Standing/AStandingGrantNeverCrossesItsFloorsTest.php
index c1c052d..f20ee48 100644
--- a/tests/Standing/AStandingGrantNeverCrossesItsFloorsTest.php
+++ b/tests/Standing/AStandingGrantNeverCrossesItsFloorsTest.php
@@ -100,8 +100,8 @@ public static function documentsThatAreRefused(): iterable
             ['name' => 'c', 'statement' => 't', 'by' => ['operation' => 'lab:build']],
         ]], 'each under its own name'];
         yield 'conditions not given as a list' => [['conditions' => ['c' => ['name' => 'c']]], 'gives «conditions» as a list'];
-        yield 'an expiry that is not a date' => [['expires_at' => 'next week, maybe'], 'gives «expires_at» as a date and time'];
-        yield 'an expiry that is not text' => [['expires_at' => 1790000000], 'gives «expires_at» as a date and time'];
+        yield 'an expiry that is not a date' => [['expires_at' => 'next week, maybe'], 'gives «expires_at» as a date and time with its offset, in ISO 8601'];
+        yield 'an expiry that is not text' => [['expires_at' => 1790000000], 'gives «expires_at» as a date and time with its offset, in ISO 8601'];
     }
 
     public function testTheValuesRefuseTheSameThingsWhenBuiltByHand(): void