diff --git a/.changes/nextrelease/expires-customization.json b/.changes/nextrelease/expires-customization.json
new file mode 100644
index 0000000000..2761e2a8ec
--- /dev/null
+++ b/.changes/nextrelease/expires-customization.json
@@ -0,0 +1,7 @@
+[
+  {
+    "type": "feature",
+    "category": "S3",
+    "description": "Adds customization to output structures for `Expires` parsing which adds an additional shape `ExpiresString`"
+  }
+]
diff --git a/src/Api/Service.php b/src/Api/Service.php
index 0945d92faa..52cc731776 100644
--- a/src/Api/Service.php
+++ b/src/Api/Service.php
@@ -284,7 +284,7 @@ public function getOperation($name)
                 $this->definition['operations'][$name],
                 $this->shapeMap
             );
-        } else if ($this->modifiedModel) {
+        } elseif ($this->modifiedModel) {
             $this->operations[$name] = new Operation(
                 $this->definition['operations'][$name],
                 $this->shapeMap
@@ -517,6 +517,7 @@ public function getDefinition()
     public function setDefinition($definition)
     {
         $this->definition = $definition;
+        $this->shapeMap = new ShapeMap($definition['shapes']);
         $this->modifiedModel = true;
     }
 
diff --git a/src/InputValidationMiddleware.php b/src/InputValidationMiddleware.php
index bd46582df2..f1933d6dfb 100644
--- a/src/InputValidationMiddleware.php
+++ b/src/InputValidationMiddleware.php
@@ -72,5 +72,4 @@ public function __invoke(CommandInterface $cmd) {
         }
         return $nextHandler($cmd);
     }
-
 }
diff --git a/src/S3/ExpiresParsingMiddleware.php b/src/S3/ExpiresParsingMiddleware.php
new file mode 100644
index 0000000000..5ceccd1eaa
--- /dev/null
+++ b/src/S3/ExpiresParsingMiddleware.php
@@ -0,0 +1,56 @@
+<?php
+namespace Aws\S3;
+
+use Aws\CommandInterface;
+use Aws\ResultInterface;
+use Psr\Http\Message\RequestInterface;
+
+/**
+ * Logs a warning when the `expires` header
+ * fails to be parsed.
+ *
+ * @internal
+ */
+class ExpiresParsingMiddleware
+{
+    /** @var callable  */
+    private $nextHandler;
+
+    /**
+     * Create a middleware wrapper function.
+     *
+     * @return callable
+     */
+    public static function wrap()
+    {
+        return function (callable $handler) {
+            return new self($handler);
+        };
+    }
+
+    /**
+     * @param callable $nextHandler Next handler to invoke.
+     */
+    public function __construct(callable $nextHandler)
+    {
+        $this->nextHandler = $nextHandler;
+    }
+
+    public function __invoke(CommandInterface $command, RequestInterface $request = null)
+    {
+        $next = $this->nextHandler;
+        return $next($command, $request)->then(
+            function (ResultInterface $result) {
+                if (empty($result['Expires']) && !empty($result['ExpiresString'])) {
+                    trigger_error(
+                        "Failed to parse the `expires` header as a timestamp due to "
+                        . " an invalid timestamp format.\nPlease refer to `ExpiresString` "
+                        . "for the unparsed string format of this header.\n"
+                        , E_USER_WARNING
+                    );
+                }
+                return $result;
+            }
+        );
+    }
+}
diff --git a/src/S3/S3Client.php b/src/S3/S3Client.php
index 235bac3b4c..2948802ff2 100644
--- a/src/S3/S3Client.php
+++ b/src/S3/S3Client.php
@@ -437,6 +437,7 @@ public function __construct(array $args)
             InputValidationMiddleware::wrap($this->getApi(), self::$mandatoryAttributes),
             'input_validation_middleware'
         );
+        $stack->appendSign(ExpiresParsingMiddleware::wrap(), 's3.expires_parsing');
         $stack->appendSign(PutObjectUrlMiddleware::wrap(), 's3.put_object_url');
         $stack->appendSign(PermanentRedirectMiddleware::wrap(), 's3.permanent_redirect');
         $stack->appendInit(Middleware::sourceFile($this->getApi()), 's3.source_file');
@@ -444,8 +445,8 @@ public function __construct(array $args)
         $stack->appendInit($this->getLocationConstraintMiddleware(), 's3.location');
         $stack->appendInit($this->getEncodingTypeMiddleware(), 's3.auto_encode');
         $stack->appendInit($this->getHeadObjectMiddleware(), 's3.head_object');
+        $this->processModel($this->isUseEndpointV2());
         if ($this->isUseEndpointV2()) {
-            $this->processEndpointV2Model();
             $stack->after('builder',
                 's3.check_empty_path_with_query',
                 $this->getEmptyPathWithQuery());
@@ -763,28 +764,51 @@ public static function _default_s3_express_identity_provider(array $args)
     }
 
     /**
-     * Modifies API definition to remove `Bucket` from request URIs.
+     * If EndpointProviderV2 is used, removes `Bucket` from request URIs.
      * This is now handled by the endpoint ruleset.
      *
+     * Additionally adds a synthetic shape `ExpiresString` and modifies
+     * `Expires` type to ensure it remains set to `timestamp`.
+     *
+     * @param array $args
      * @return void
      *
      * @internal
      */
-    private function processEndpointV2Model()
+    private function processModel(bool $isUseEndpointV2): void
     {
         $definition = $this->getApi()->getDefinition();
 
-        foreach($definition['operations'] as &$operation) {
-            if (isset($operation['http']['requestUri'])) {
-                $requestUri = $operation['http']['requestUri'];
-                if ($requestUri === "/{Bucket}") {
-                    $requestUri = str_replace('/{Bucket}', '/', $requestUri);
-                } else {
-                    $requestUri = str_replace('/{Bucket}', '', $requestUri);
+        if ($isUseEndpointV2) {
+            foreach($definition['operations'] as &$operation) {
+                if (isset($operation['http']['requestUri'])) {
+                    $requestUri = $operation['http']['requestUri'];
+                    if ($requestUri === "/{Bucket}") {
+                        $requestUri = str_replace('/{Bucket}', '/', $requestUri);
+                    } else {
+                        $requestUri = str_replace('/{Bucket}', '', $requestUri);
+                    }
+                    $operation['http']['requestUri'] = $requestUri;
+                }
+            }
+        }
+
+        foreach ($definition['shapes'] as $key => &$value) {
+            $suffix = 'Output';
+            if (substr($key, -strlen($suffix)) === $suffix) {
+                if (isset($value['members']['Expires'])) {
+                    $value['members']['Expires']['deprecated'] = true;
+                    $value['members']['ExpiresString'] = [
+                        'shape' => 'ExpiresString',
+                        'location' => 'header',
+                        'locationName' => 'Expires'
+                    ];
                 }
-                $operation['http']['requestUri'] = $requestUri;
             }
         }
+        $definition['shapes']['ExpiresString']['type'] = 'string';
+        $definition['shapes']['Expires']['type'] = 'timestamp';
+
         $this->getApi()->setDefinition($definition);
     }
 
@@ -1035,6 +1059,29 @@ public static function applyDocFilters(array $api, array $docs)
             'shapes' => ['PutObjectRequest', 'UploadPartRequest']
         ];
 
+        // Add `ExpiresString` shape to output structures which contain `Expires`
+        // Deprecate existing `Expires` shapes in output structures
+        // Add/Update documentation for both `ExpiresString` and `Expires`
+        // Ensure `Expires` type remains timestamp
+        foreach ($api['shapes'] as $key => &$value) {
+            $suffix = 'Output';
+            if (substr($key, -strlen($suffix)) === $suffix) {
+                if (isset($value['members']['Expires'])) {
+                    $value['members']['Expires']['deprecated'] = true;
+                    $value['members']['ExpiresString'] = [
+                        'shape' => 'ExpiresString',
+                        'location' => 'header',
+                        'locationName' => 'Expires'
+                    ];
+                    $docs['shapes']['Expires']['refs'][$key . '$Expires']
+                        .= '<p>This output shape has been deprecated. Please refer to <code>ExpiresString</code> instead.</p>.';
+                }
+            }
+        }
+        $api['shapes']['ExpiresString']['type'] = 'string';
+        $docs['shapes']['ExpiresString']['base'] = 'The unparsed string value of the <code>Expires</code> output member.';
+        $api['shapes']['Expires']['type'] = 'timestamp';
+
         return [
             new Service($api, ApiProvider::defaultProvider()),
             new DocModel($docs)
diff --git a/tests/Api/ServiceTest.php b/tests/Api/ServiceTest.php
index 78c9b67bd6..08c9b187c6 100644
--- a/tests/Api/ServiceTest.php
+++ b/tests/Api/ServiceTest.php
@@ -269,14 +269,40 @@ public function testModifyModel()
                     'signingName'     => 'qux',
                     'protocol'        => 'yak',
                     'uid'             => 'foo-2016-12-09'
+                ],
+                'operations' => [
+                    'FooOperation' => [
+                        'name' => 'FooOperation',
+                        'output' => [
+                            'shape' => 'FooOperationOutput'
+                        ]
+                    ]
+                ],
+                'shapes' => [
+                    'FooOperationOutput' => [
+                        'type' => 'structure',
+                        'members' => [
+                            'Expires' => [
+                                'shape' => 'Expires',
+                            ]
+                        ]
+                    ],
+                    'Expires' => [
+                        'type' => 'string'
+                    ]
                 ]
             ],
             function () { return []; }
         );
         $definition = $s->getDefinition();
         $definition['metadata']['serviceId'] = 'bar';
+        $definition['shapes']['Expires']['type'] = 'timestamp';
         $s->setDefinition($definition);
         $this->assertTrue($s->isModifiedModel());
         $this->assertEquals( 'bar', $s->getMetadata('serviceId'));
+        $this->assertEquals(
+            'timestamp',
+            $s->getOperation('FooOperation')->getOutput()->getMember('Expires')->getType()
+        );
     }
 }
diff --git a/tests/S3/ExpiresParsingMiddlewareTest.php b/tests/S3/ExpiresParsingMiddlewareTest.php
new file mode 100644
index 0000000000..6b1aaa6e99
--- /dev/null
+++ b/tests/S3/ExpiresParsingMiddlewareTest.php
@@ -0,0 +1,56 @@
+<?php
+namespace Aws\Test\S3;
+
+use Aws\CommandInterface;
+use Aws\Result;
+use Aws\S3\ExpiresParsingMiddleware;
+use Aws\Test\UsesServiceTrait;
+use GuzzleHttp\Promise;
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
+use Psr\Http\Message\RequestInterface;
+
+/**
+ * @covers Aws\S3\ExpiresParsingMiddleware
+ */
+class ExpiresParsingMiddlewareTest extends TestCase
+{
+    use UsesServiceTrait;
+
+    public function testEmitsWarningWhenMissingExpires()
+    {
+        $this->expectWarning();
+        $this->expectWarningMessage(
+            "Failed to parse the `expires` header as a timestamp due to "
+            . " an invalid timestamp format.\nPlease refer to `ExpiresString` "
+            . "for the unparsed string format of this header.\n"
+        );
+
+        $command = $this->getMockBuilder(CommandInterface::class)->getMock();
+        $request = $this->getMockBuilder(RequestInterface::class)->getMock();
+        $nextHandler = function ($cmd, $request) {
+            return Promise\Create::promiseFor(new Result([
+                'ExpiresString' => 'not-a-timestamp'
+            ]));
+        };
+
+        $mw = new ExpiresParsingMiddleware($nextHandler);
+        $mw($command, $request)->wait();
+    }
+
+    public function testDoesNotEmitWarningWhenExpiresPresent()
+    {
+        $command = $this->getMockBuilder(CommandInterface::class)->getMock();
+        $request = $this->getMockBuilder(RequestInterface::class)->getMock();
+        $nextHandler = function ($cmd, $request) {
+            return Promise\Create::promiseFor(new Result([
+                'ExpiresString' => 'test',
+                'Expires' => 'test'
+            ]));
+        };
+
+        $mw = new ExpiresParsingMiddleware($nextHandler);
+        $result = $mw($command, $request)->wait();
+        $this->assertEquals('test', $result['Expires']);
+        $this->assertEquals('test', $result['ExpiresString']);
+    }
+}
diff --git a/tests/S3/S3ClientTest.php b/tests/S3/S3ClientTest.php
index 7eddeebfc6..d78ca73250 100644
--- a/tests/S3/S3ClientTest.php
+++ b/tests/S3/S3ClientTest.php
@@ -1,6 +1,7 @@
 <?php
 namespace Aws\Test\S3;
 
+use Aws\Api\DateTimeResult;
 use Aws\Command;
 use Aws\CommandInterface;
 use Aws\Exception\AwsException;
@@ -2497,6 +2498,86 @@ public function testCorrectlyResolvesGlobalEndpointWithoutRegionInConstructor(
         putenv('AWS_REGION=');
     }
 
+    public function testExpiresStringInResult()
+    {
+        $client = new S3Client([
+            'region' => 'us-east-1',
+            'http_handler' => function (RequestInterface $request) {
+                return Promise\Create::promiseFor(new Response(
+                    200,
+                    ['expires' => '1989-08-05']
+                ));
+            },
+        ]);
+        $result = $client->headObject(['Bucket' => 'foo', 'Key' => 'bar']);
+        $this->assertInstanceOf(DateTimeResult::class, $result['Expires']);
+        $this->assertEquals('1989-08-05', $result['ExpiresString']);
+    }
+
+    public function testEmitsWarningWhenExpiresUnparseable()
+    {
+        $this->expectWarning();
+        $this->expectWarningMessage(
+            "Failed to parse the `expires` header as a timestamp due to "
+            . " an invalid timestamp format.\nPlease refer to `ExpiresString` "
+            . "for the unparsed string format of this header.\n"
+        );
+
+        $client = new S3Client([
+            'region' => 'us-east-1',
+            'http_handler' => function (RequestInterface $request) {
+                return Promise\Create::promiseFor(new Response(
+                    200,
+                    ['expires' => 'this-is-not-a-timestamp']
+                ));
+            },
+        ]);
+
+        $client->headObject(['Bucket' => 'foo', 'Key' => 'bar']);
+    }
+
+    public function testExpiresRemainsTimestamp() {
+        //S3 will be changing `Expires` type from `timestamp` to `string`
+        // soon.  This test ensures backward compatibility
+        $apiProvider = static function () {
+            return [
+                'metadata' => [
+                    'signatureVersion' => 'v4',
+                    'protocol' => 'rest-xml'
+                ],
+                'shapes' => [
+                    'Expires' => [
+                        'type' => 'string'
+                    ],
+                ],
+            ];
+        };
+
+        $s3Client = new S3Client([
+            'region' => 'us-west-2',
+            'api_provider' => $apiProvider
+        ]);
+
+        $api = $s3Client->getApi();
+        $expiresType = $api->getDefinition()['shapes']['Expires']['type'];
+        $this->assertEquals('timestamp', $expiresType);
+    }
+
+    public function testBucketNotModifiedWithLegacyEndpointProvider()
+    {
+        $client = new S3Client([
+            'region' => 'us-west-2',
+            'endpoint_provider' => PartitionEndpointProvider::defaultProvider()
+        ]);
+
+        $operations = $client->getApi()->getDefinition()['operations'];
+        $this->assertEquals('/{Bucket}', $operations['ListObjects']['http']['requestUri']);
+        $this->assertEquals(
+            '/{Bucket}?versions',
+            $operations['ListObjectVersions']['http']['requestUri']
+        );
+    }
+
     public function builtinRegionProvider()
     {
         return [