date: Fix Error message missing '+' for format string after valid d flag

src/uu/date/locales/en-US.ftl

@@ -106,3 +106,6 @@ date-error-setting-date-not-supported-redox = setting the date is not supported
 date-error-cannot-set-date = cannot set date
 date-error-extra-operand = extra operand '{$operand}'
 date-error-write = write error: {$error}
+date-error-format-missing-plus = the argument {$arg} lacks a leading '+';
+  when using an option to specify date(s), any non-option
+  argument must be a format string beginning with '+'

src/uu/date/locales/fr-FR.ftl

@@ -101,3 +101,6 @@ date-error-setting-date-not-supported-redox = la définition de la date n'est pa
 date-error-cannot-set-date = impossible de définir la date
 date-error-extra-operand = opérande supplémentaire '{$operand}'
 date-error-write = erreur d'écriture: {$error}
+date-error-format-missing-plus = l'argument {$arg} ne commence pas par un signe '+';
+ lorsqu'une option est utilisée pour spécifier une ou plusieurs dates, tout argument autre 
+  qu'une option doit être une chaîne de format commençant par un signe '+'.

src/uu/date/src/date.rs

@@ -285,6 +285,27 @@ fn parse_military_timezone_with_offset(s: &str) -> Option<(i32, DayDelta)> {
 pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     let matches = uucore::clap_localization::handle_clap_result(uu_app(), args)?;
 
+    let date_source = if let Some(date_os) = matches.get_one::<std::ffi::OsString>(OPT_DATE) {
+        // Convert OsString to String, handling invalid UTF-8 with GNU-compatible error
+        let date = date_os.to_str().ok_or_else(|| {
+            let bytes = date_os.as_encoded_bytes();
+            let escaped_str = escape_invalid_bytes(bytes);
+            USimpleError::new(1, format!("invalid date '{escaped_str}'"))
+        })?;
+        DateSource::Human(date.into())
+    } else if let Some(file) = matches.get_one::<String>(OPT_FILE) {
+        match file.as_ref() {
+            "-" => DateSource::Stdin,
+            _ => DateSource::File(file.into()),
+        }
+    } else if let Some(file) = matches.get_one::<String>(OPT_REFERENCE) {
+        DateSource::FileMtime(file.into())
+    } else if matches.get_flag(OPT_RESOLUTION) {
+        DateSource::Resolution
+    } else {
+        DateSource::Now
+    };
+
     // Check for extra operands (multiple positional arguments)
     if let Some(formats) = matches.get_many::<String>(OPT_FORMAT) {
         let format_args: Vec<&String> = formats.collect();
@@ -298,9 +319,19 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
 
     let format = if let Some(form) = matches.get_one::<String>(OPT_FORMAT) {
         if !form.starts_with('+') {
+            // if an optional Format String was found but the user has not provided an input date
+            // GNU prints an invalid date Error
+            if !matches!(date_source, DateSource::Human(_)) {
+                return Err(USimpleError::new(
+                    1,
+                    translate!("date-error-invalid-date", "date" => form),
+                ));
+            }
+            // If the user did provide an input date with the --date flag and the Format String is
+            // not starting with '+' GNU prints the missing '+' error message
             return Err(USimpleError::new(
                 1,
-                translate!("date-error-invalid-date", "date" => form),
+                translate!("date-error-format-missing-plus", "arg" => form),
             ));
         }
         let form = form[1..].to_string();
@@ -333,27 +364,6 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         Zoned::now()
     };
 
-    let date_source = if let Some(date_os) = matches.get_one::<std::ffi::OsString>(OPT_DATE) {
-        // Convert OsString to String, handling invalid UTF-8 with GNU-compatible error
-        let date = date_os.to_str().ok_or_else(|| {
-            let bytes = date_os.as_encoded_bytes();
-            let escaped_str = escape_invalid_bytes(bytes);
-            USimpleError::new(1, format!("invalid date '{escaped_str}'"))
-        })?;
-        DateSource::Human(date.into())
-    } else if let Some(file) = matches.get_one::<String>(OPT_FILE) {
-        match file.as_ref() {
-            "-" => DateSource::Stdin,
-            _ => DateSource::File(file.into()),
-        }
-    } else if let Some(file) = matches.get_one::<String>(OPT_REFERENCE) {
-        DateSource::FileMtime(file.into())
-    } else if matches.get_flag(OPT_RESOLUTION) {
-        DateSource::Resolution
-    } else {
-        DateSource::Now
-    };
-
     let set_to = match matches
         .get_one::<String>(OPT_SET)
         .map(|s| parse_date(s, &now, DebugOptions::new(debug_mode, true)))

tests/by-util/test_date.rs

@@ -35,6 +35,18 @@ fn test_extra_operands() {
         .stderr_contains("extra operand 'extra'");
 }
 
+#[test]
+fn test_bad_format_option_missing_leading_plus_after_d_flag() {
+    let bad_arguments = vec!["q", "a", "test", "%Y-%m-%d"];
+
+    for bad_argument in bad_arguments {
+        new_ucmd!()
+            .args(&["--date", "1996-01-31", bad_argument])
+            .fails_with_code(1)
+            .stderr_contains(format!("the argument {bad_argument} lacks a leading '+';\nwhen using an option to specify date(s), any non-option\nargument must be a format string beginning with '+'"), );
+    }
+}
+
 #[test]
 fn test_invalid_long_option() {
     new_ucmd!()