diff --git a/format.go b/format.go
index a9fff05..58342b7 100644
--- a/format.go
+++ b/format.go
@@ -21,15 +21,15 @@ var builtInNumFmt = map[uint16]string{
 	11: "0.00e+00",
 	12: "# ?/?",
 	13: "# ??/??",
-	14: "mm-dd-yy",
-	15: "d-mmm-yy",
+	14: "mm-dd-yyyy",
+	15: "d-mmm-yyyy",
 	16: "d-mmm",
-	17: "mmm-yy",
+	17: "mmm-yyyy",
 	18: "h:mm am/pm",
 	19: "h:mm:ss am/pm",
 	20: "h:mm",
 	21: "h:mm:ss",
-	22: "m/d/yy h:mm",
+	22: "m/d/yyyy h:mm",
 	37: "#,##0 ;(#,##0)",
 	38: "#,##0 ;[red](#,##0)",
 	39: "#,##0.00;(#,##0.00)",