diff --git a/material/plugins/blog/plugin.py b/material/plugins/blog/plugin.py
index 375b8cfe7..7d2e5595f 100644
--- a/material/plugins/blog/plugin.py
+++ b/material/plugins/blog/plugin.py
@@ -800,7 +800,7 @@ class BlogPlugin(BasePlugin[BlogConfig]):
 
     # Format date
     def _format_date(self, date: datetime, format: str, config: MkDocsConfig):
-        locale = config.theme["language"]
+        locale: str = config.theme["language"].replace("-", "_")
         return format_date(date, format = format, locale = locale)
 
     # Format date for post
diff --git a/src/plugins/blog/plugin.py b/src/plugins/blog/plugin.py
index 375b8cfe7..7d2e5595f 100644
--- a/src/plugins/blog/plugin.py
+++ b/src/plugins/blog/plugin.py
@@ -800,7 +800,7 @@ class BlogPlugin(BasePlugin[BlogConfig]):
 
     # Format date
     def _format_date(self, date: datetime, format: str, config: MkDocsConfig):
-        locale = config.theme["language"]
+        locale: str = config.theme["language"].replace("-", "_")
         return format_date(date, format = format, locale = locale)
 
     # Format date for post