diff --git a/source/detail/number_format/number_formatter.cpp b/source/detail/number_format/number_formatter.cpp
index 485a8b63..82341d35 100644
--- a/source/detail/number_format/number_formatter.cpp
+++ b/source/detail/number_format/number_formatter.cpp
@@ -466,6 +466,8 @@ const std::unordered_map<int, std::string> known_locales()
             {0x7C68, "Hausa (Latin)"},
             {0x7C86, "K’iche’"},
             {0x7C92, "Central Kurdish (Arabic)"},
+            {0xF400, "System Default for Time"},
+            {0xF800, "System Default for Long Date"},
         });
 
     return all;
diff --git a/source/detail/number_format/number_formatter.hpp b/source/detail/number_format/number_formatter.hpp
index cfb00a1c..4267d69e 100644
--- a/source/detail/number_format/number_formatter.hpp
+++ b/source/detail/number_format/number_formatter.hpp
@@ -530,7 +530,9 @@ enum class format_locale
     tamazight_latin = 0x7C5F,
     fulah_latin = 0x7C67,
     hausa_latin = 0x7C68,
-    central_kurdish_arabic = 0x7C92
+    central_kurdish_arabic = 0x7C92,
+    system_default_time = 0xF400,
+    system_default_long_date = 0xF800
 };
 
 // TODO this really shouldn't be exported...