fix: match date -I error messages with GNU date (#4499)

* date: use value_parser to improve errors

* date: update panic! to unreachable inside impl From for Iso8601Format

src/uu/date/src/date.rs

@@ -114,8 +114,8 @@ impl<'a> From<&'a str> for Iso8601Format {
             SECONDS | SECOND => Self::Seconds,
             NS => Self::Ns,
             DATE => Self::Date,
-            // Should be caught by clap
+            // Note: This is caught by clap via `possible_values`
-            _ => panic!("Invalid format: {s}"),
+            _ => unreachable!(),
         }
     }
 }
@@ -291,6 +291,7 @@ pub fn uu_app() -> Command {
                 .short('I')
                 .long(OPT_ISO_8601)
                 .value_name("FMT")
+                .value_parser([DATE, HOUR, HOURS, MINUTE, MINUTES, SECOND, SECONDS, NS])
                 .help(ISO_8601_HELP_STRING),
         )
         .arg(

tests/by-util/test_date.rs

@@ -50,6 +50,13 @@ fn test_date_rfc_8601() {
     }
 }
 
+#[test]
+fn test_date_rfc_8601_invalid_arg() {
+    for param in ["--iso-8601", "--i"] {
+        new_ucmd!().arg(format!("{param}=@")).fails();
+    }
+}
+
 #[test]
 fn test_date_rfc_8601_second() {
     for param in ["--iso-8601", "--i"] {