date: fix inconsistent input parsing between -s and -d flags (#9690)

Add allow_hyphen_values(true) to -s flag to accept hyphen-prefixed values like '-3 days', making it consistent with -d flag behavior and GNU coreutils compatibility.

Fixes #9679

src/uu/date/src/date.rs

@@ -487,6 +487,7 @@ pub fn uu_app() -> Command {
                 .short('s')
                 .long(OPT_SET)
                 .value_name("STRING")
+                .allow_hyphen_values(true)
                 .help({
                     #[cfg(not(any(target_os = "macos", target_os = "redox")))]
                     {

tests/by-util/test_date.rs

@@ -293,6 +293,27 @@ fn test_date_set_permissions_error() {
     }
 }
 
+#[test]
+#[cfg(all(unix, not(any(target_os = "android", target_os = "macos"))))]
+fn test_date_set_hyphen_prefixed_values() {
+    // test -s flag accepts hyphen-prefixed values like "-3 days"
+    if !(geteuid() == 0 || uucore::os::is_wsl_1()) {
+        let test_cases = vec!["-1 hour", "-2 days", "-3 weeks", "-1 month"];
+
+        for date_str in test_cases {
+            let result = new_ucmd!().arg("--set").arg(date_str).fails();
+            result.no_stdout();
+            // permission error, not argument parsing error
+            assert!(
+                result.stderr_str().starts_with("date: cannot set date: "),
+                "Expected permission error for '{}', but got: {}",
+                date_str,
+                result.stderr_str()
+            );
+        }
+    }
+}
+
 #[test]
 #[cfg(target_os = "macos")]
 fn test_date_set_mac_unavailable() {