diff --git a/src/md-export.ts b/src/md-export.ts
index 50897f5..5ffb8ca 100644
--- a/src/md-export.ts
+++ b/src/md-export.ts
@@ -33,8 +33,20 @@ function bookmarksDir(): string {
   return path.join(mdDir(), 'bookmarks');
 }
 
+function formatSafeDate(dateStr: string): string {
+  const parsed = Date.parse(dateStr);
+  if (Number.isFinite(parsed)) {
+    const d = new Date(parsed);
+    const yyyy = d.getFullYear();
+    const mm = String(d.getMonth() + 1).padStart(2, '0');
+    const dd = String(d.getDate()).padStart(2, '0');
+    return `${yyyy}-${mm}-${dd}`;
+  }
+  return 'undated';
+}
+
 function bookmarkFilename(b: BookmarkTimelineItem): string {
-  const date = (b.postedAt ?? b.bookmarkedAt ?? '').slice(0, 10) || 'undated';
+  const date = formatSafeDate(b.postedAt ?? b.bookmarkedAt ?? '');
   const author = b.authorHandle ? slug(b.authorHandle) : 'unknown';
   const textSlug = slug(b.text.slice(0, 50)) || b.id;
   return `${date}-${author}-${textSlug}.md`;