diff --git a/packages/storage/__tests__/providers/s3/utils/client/S3/cases/copyObject.ts b/packages/storage/__tests__/providers/s3/utils/client/S3/cases/copyObject.ts
index c20c0394a0e..8938a2ce9c8 100644
--- a/packages/storage/__tests__/providers/s3/utils/client/S3/cases/copyObject.ts
+++ b/packages/storage/__tests__/providers/s3/utils/client/S3/cases/copyObject.ts
@@ -37,7 +37,7 @@ const copyObjectHappyCase: ApiFunctionalTestCase<typeof copyObject> = [
 			'content-type': 'contentType',
 			'x-amz-acl': 'acl',
 			'x-amz-copy-source-if-match': 'eTag',
-			'x-amz-copy-source-if-unmodified-since': '1970-01-01T00:00:00.000Z',
+			'x-amz-copy-source-if-unmodified-since': 'Thu, 01 Jan 1970 00:00:00 GMT',
 		}),
 	}),
 	{
diff --git a/packages/storage/__tests__/providers/s3/utils/client/s3Data/copyObject.test.ts b/packages/storage/__tests__/providers/s3/utils/client/s3Data/copyObject.test.ts
index bef1281e62c..01ce54eb16f 100644
--- a/packages/storage/__tests__/providers/s3/utils/client/s3Data/copyObject.test.ts
+++ b/packages/storage/__tests__/providers/s3/utils/client/s3Data/copyObject.test.ts
@@ -114,7 +114,8 @@ describe('validateCopyObjectHeaders', () => {
 				...baseHeaders,
 				'x-amz-metadata-directive': 'mock-metadata',
 				'x-amz-copy-source-if-match': 'mock-etag',
-				'x-amz-copy-source-if-unmodified-since': '1970-01-01T00:00:00.000Z',
+				'x-amz-copy-source-if-unmodified-since':
+					'Thu, 01 Jan 1970 00:00:00 GMT',
 			},
 			expectPass: true,
 		},
@@ -125,7 +126,8 @@ describe('validateCopyObjectHeaders', () => {
 				...baseHeaders,
 				'x-amz-metadata-directive': 'mock-metadata',
 				'x-amz-copy-source-if-match': 'mock-etag',
-				'x-amz-copy-source-if-unmodified-since': '1970-01-01T00:00:00.000Z',
+				'x-amz-copy-source-if-unmodified-since':
+					'Thu, 01 Jan 1970 00:00:00 GMT',
 			},
 			expectPass: false,
 		},
diff --git a/packages/storage/src/providers/s3/utils/client/s3data/copyObject.ts b/packages/storage/src/providers/s3/utils/client/s3data/copyObject.ts
index 3e65ab03671..f1dd2784d13 100644
--- a/packages/storage/src/providers/s3/utils/client/s3data/copyObject.ts
+++ b/packages/storage/src/providers/s3/utils/client/s3data/copyObject.ts
@@ -59,7 +59,7 @@ const copyObjectSerializer = async (
 			'x-amz-metadata-directive': input.MetadataDirective,
 			'x-amz-copy-source-if-match': input.CopySourceIfMatch,
 			'x-amz-copy-source-if-unmodified-since':
-				input.CopySourceIfUnmodifiedSince?.toISOString(),
+				input.CopySourceIfUnmodifiedSince?.toUTCString(),
 			'x-amz-source-expected-bucket-owner': input.ExpectedSourceBucketOwner,
 			'x-amz-expected-bucket-owner': input.ExpectedBucketOwner,
 		}),
@@ -96,7 +96,7 @@ export const validateCopyObjectHeaders = (
 			headers['x-amz-copy-source-if-match'],
 		),
 		bothNilOrEqual(
-			input.CopySourceIfUnmodifiedSince?.toISOString(),
+			input.CopySourceIfUnmodifiedSince?.toUTCString(),
 			headers['x-amz-copy-source-if-unmodified-since'],
 		),
 	];