[YoutubeDL] Fix output template for missing timestamp (closes #12796)

youtube_dl/YoutubeDL.py

@@ -640,7 +640,7 @@ class YoutubeDL(object):
 
             NUMERIC_FIELDS = set((
                 'width', 'height', 'tbr', 'abr', 'asr', 'vbr', 'fps', 'filesize', 'filesize_approx',
-                'upload_year', 'upload_month', 'upload_day',
+                'timestamp', 'upload_year', 'upload_month', 'upload_day',
                 'duration', 'view_count', 'like_count', 'dislike_count', 'repost_count',
                 'average_rating', 'comment_count', 'age_limit',
                 'start_time', 'end_time',