diff --git a/src/v4/aliyun_util.rs b/src/v4/aliyun_util.rs
index 337fa91..08daf4a 100644
--- a/src/v4/aliyun_util.rs
+++ b/src/v4/aliyun_util.rs
@@ -51,7 +51,6 @@ fn add_common_headers(header: &mut BTreeMap<String, String>, request: &Request)
         }
     }
 
-    let ymd = "yyyymmdd";
     let query = BTreeMap::new();
 
     if let Some(access_key) = &request.access_key {
@@ -63,7 +62,7 @@ fn add_common_headers(header: &mut BTreeMap<String, String>, request: &Request)
         let signing_key = get_signing_key(
             &request.sign_algorithm,
             &access_key.access_key_secret,
-            ymd,
+            &ymd,
             &request.region,
             &request.product,
         );
@@ -74,7 +73,7 @@ fn add_common_headers(header: &mut BTreeMap<String, String>, request: &Request)
         let authorization = get_authorization(
             &request.sign_algorithm,
             &derived_access_key,
-            ymd,
+            &ymd,
             &request.region,
             &request.product,
             &request.pathname,