diff --git a/ios/Classes/SwiftPokepaySdkPlugin.swift b/ios/Classes/SwiftPokepaySdkPlugin.swift
index 9a1801d..5bf4ef2 100644
--- a/ios/Classes/SwiftPokepaySdkPlugin.swift
+++ b/ios/Classes/SwiftPokepaySdkPlugin.swift
@@ -11,6 +11,12 @@ private class APIJSONEncoder : JSONEncoder, @unchecked Sendable {
         let dateFormatter = DateFormatter()
         dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSSSS'Z'"
         dateFormatter.timeZone = TimeZone(secondsFromGMT: 0)
+        // A fixed-format DateFormatter must always use en_US_POSIX (Apple QA1480).
+        // Otherwise Locale.current is used, and Foundation rewrites "HH" into
+        // "h" plus an AM/PM marker when the device is set to 12-hour time. The
+        // date handed to Flutter then stops being ISO8601 and DateTime.parse
+        // throws a FormatException on the Dart side.
+        dateFormatter.locale = Locale(identifier: "en_US_POSIX")
         dateEncodingStrategy = .formatted(dateFormatter)
     }
 }
diff --git a/test/responses/check_test.dart b/test/responses/check_test.dart
new file mode 100644
index 0000000..c16619a
--- /dev/null
+++ b/test/responses/check_test.dart
@@ -0,0 +1,138 @@
+import 'package:test/test.dart';
+
+import 'package:pokepay_sdk/responses/check.dart';
+import 'package:pokepay_sdk/responses/token_info.dart';
+
+/// Mimics the Check JSON the native side pushes over the MethodChannel
+/// (iOS: APIJSONEncoder in SwiftPokepaySdkPlugin, Android: Response.toString
+/// in pokepaylib). Only expires_at varies, so each test can check how a
+/// particular date format is handled.
+Map<String, dynamic> checkJson(String expiresAt) => <String, dynamic>{
+      'id': 'b3f04a3f-0a17-42a4-85f1-1376a4f07c07',
+      'amount': 1000.0,
+      'money_amount': 1000.0,
+      'point_amount': 0.0,
+      'description': 'topup QR',
+      'user': <String, dynamic>{
+        'id': '4b4a2d5a-4d5c-4c1a-9c2f-2c5c9a5b1d3e',
+        'name': 'test user',
+        'is_merchant': false,
+      },
+      'private_money': <String, dynamic>{
+        'id': '9f1a0c33-6b4e-4a52-8d4a-1a2b3c4d5e6f',
+        'name': 'test money',
+        'type': 'own',
+        'unit': '円',
+        'description': 'money for tests',
+        'oneline_message': '',
+        'account_image': null,
+        'images': <String, dynamic>{
+          'card': null,
+          '300x300': null,
+          '600x600': null,
+        },
+        'organization': <String, dynamic>{
+          'code': 'test',
+          'name': 'test organization',
+        },
+        'max_balance': null,
+        'transfer_limit': null,
+        'expiration_type': 'unlimited',
+        'is_exclusive': false,
+        'terms_url': null,
+        'privacy_policy_url': null,
+        'payment_act_url': null,
+        'commercial_act_url': null,
+        'can_use_credit_card': false,
+        'can_use_c2c_transfer': false,
+        'custom_domain_name': null,
+        'topup_methods': null,
+      },
+      'is_onetime': true,
+      'is_disabled': false,
+      'expires_at': expiresAt,
+      'point_expires_at': null,
+      'point_expires_in_days': null,
+      'token': 'b3f04a3f-0a17-42a4-85f1-1376a4f07c07',
+    };
+
+void main() {
+  group('Check.fromJson expires_at', () {
+    test('parses the ISO8601 the server returns (6-digit microseconds)', () {
+      final check = Check.fromJson(checkJson('2026-11-20T06:56:43.091034Z'));
+
+      expect(check.expiresAt.isUtc, isTrue);
+      expect(
+        check.expiresAt.toUtc().toIso8601String(),
+        '2026-11-20T06:56:43.091034Z',
+      );
+    });
+
+    test('parses the ISO8601 the native side returns (millisecond precision '
+        'zero-padded to 6 digits)', () {
+      // The iOS APIJSONEncoder writes dates with the format
+      // "yyyy-MM-dd'T'HH:mm:ss.SSSSSS'Z'", so the last three digits are
+      // always 000.
+      final check = Check.fromJson(checkJson('2026-11-20T06:56:43.091000Z'));
+
+      // toIso8601String() omits trailing zeros, so compare the value itself.
+      expect(
+        check.expiresAt.toUtc(),
+        DateTime.utc(2026, 11, 20, 6, 56, 43, 91, 0),
+      );
+    });
+
+    // Regression tests: if the iOS DateFormatter is left without
+    // locale = en_US_POSIX, Foundation rewrites "HH" into "h" plus an AM/PM
+    // marker while the device is set to 12-hour time (Apple QA1480). The
+    // expires_at that reaches Dart then stops being ISO8601 and fails here.
+    // See APIJSONEncoder in ios/Classes/SwiftPokepaySdkPlugin.swift.
+    //
+    // The strings below are what the pre-fix APIJSONEncoder actually emitted
+    // on the iOS 26.5 simulator with Settings > General > Date & Time >
+    // 24-Hour Time turned off.
+    test('throws FormatException for the broken date a 12-hour ja_JP device '
+        'returns', () {
+      // Measured: {"expires_at":"2026-11-20T午前6:56:43.091000Z", ...}
+      expect(
+        () => Check.fromJson(checkJson('2026-11-20T午前6:56:43.091000Z')),
+        throwsA(isA<FormatException>()),
+      );
+    });
+
+    test('throws FormatException for the broken date a 12-hour en_US device '
+        'returns', () {
+      // Measured: {"expires_at":"2026-11-20T6:56:43.091000 AMZ", ...}
+      expect(
+        () => Check.fromJson(checkJson('2026-11-20T6:56:43.091000 AMZ')),
+        throwsA(isA<FormatException>()),
+      );
+    });
+  });
+
+  group('TokenInfo.fromJson (the topup QR path of getTokenInfo)', () {
+    test('restores a CHECK token as a Check', () {
+      final info = TokenInfo.fromJson(<String, dynamic>{
+        'type': 'CHECK',
+        'token': checkJson('2026-11-20T06:56:43.091034Z'),
+      });
+
+      expect(info.type, TokenType.CHECK);
+      expect(info.token, isA<Check>());
+      expect(
+        (info.token as Check).expiresAt.toUtc().toIso8601String(),
+        '2026-11-20T06:56:43.091034Z',
+      );
+    });
+
+    test('throws FormatException for a CHECK token carrying a broken date', () {
+      expect(
+        () => TokenInfo.fromJson(<String, dynamic>{
+          'type': 'CHECK',
+          'token': checkJson('2026-11-20T午前6:56:43.091000Z'),
+        }),
+        throwsA(isA<FormatException>()),
+      );
+    });
+  });
+}