fix date format (#5619)

crates/nu-command/tests/commands/date/format.rs

@@ -0,0 +1,14 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn formatter_not_valid() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        date format '%N'
+        "#
+        )
+    );
+
+    assert!(actual.err.contains("invalid format"));
+}

crates/nu-command/tests/commands/date/mod.rs

@@ -0,0 +1 @@
+mod format;

crates/nu-command/tests/commands/mod.rs

@@ -6,6 +6,7 @@ mod cal;
 mod cd;
 mod compact;
 mod cp;
+mod date;
 mod def;
 mod default;
 mod drop;