diff --git a/manifest.yaml b/manifest.yaml
index 3d2336b..c7e5071 100644
--- a/manifest.yaml
+++ b/manifest.yaml
@@ -9,8 +9,8 @@ dublin_core:
     description: ""
     format: text/markdown
     identifier: tn
-    issued: 2021-05-20
-    modified: 2021-05-20
+    issued: '2021-05-20'
+    modified: '2021-05-20'
     language:
         direction: rtl
         identifier: fa