Replace some dtype: string with timestamp[s]

README.md

@@ -75,7 +75,7 @@ dataset_info:
       - name: has_advanced_rights
         dtype: bool
       - name: date_started
-        dtype: string
+        dtype: timestamp[s]
     - name: number_of_characters
       dtype: int64
     - name: size
@@ -117,9 +117,9 @@ dataset_info:
   - name: url
     dtype: string
   - name: date_created
-    dtype: string
+    dtype: timestamp[s]
   - name: date_modified
-    dtype: string
+    dtype: timestamp[s]
   - name: is_part_of
     struct:
     - name: name