date: use --reference=file to display the file modification time

src/uu/date/src/date.rs

@@ -65,6 +65,7 @@ enum Format {
 enum DateSource {
     Now,
     File(PathBuf),
+    FileMtime(PathBuf),
     Stdin,
     Human(String),
 }
@@ -146,6 +147,8 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
             "-" => DateSource::Stdin,
             _ => DateSource::File(file.into()),
         }
+    } else if let Some(file) = matches.get_one::<String>(OPT_REFERENCE) {
+        DateSource::FileMtime(file.into())
     } else {
         DateSource::Now
     };
@@ -213,6 +216,20 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
             let iter = lines.map_while(Result::ok).map(parse_date);
             Box::new(iter)
         }
+        DateSource::FileMtime(ref path) => {
+            let metadata = std::fs::metadata(path)
+                .map_err_context(|| path.as_os_str().to_string_lossy().to_string())?;
+            let mtime = metadata.modified()?;
+            let ts = Timestamp::try_from(mtime).map_err(|e| {
+                USimpleError::new(
+                    1,
+                    translate!("date-error-cannot-set-date", "path" => path.to_string_lossy(), "error" => e),
+                )
+            })?;
+            let date = ts.to_zoned(TimeZone::try_system().unwrap_or(TimeZone::UTC));
+            let iter = std::iter::once(Ok(date));
+            Box::new(iter)
+        }
         DateSource::Now => {
             let iter = std::iter::once(Ok(now));
             Box::new(iter)

tests/by-util/test_date.rs

@@ -352,6 +352,22 @@ fn test_date_for_file() {
     ucmd.arg("--file").arg(file).succeeds();
 }
 
+#[test]
+fn test_date_for_file_mtime() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let file = "reference_file";
+    at.touch(file);
+    std::thread::sleep(std::time::Duration::from_millis(100));
+    let result = ucmd.arg("--reference").arg(file).arg("+%s%N").succeeds();
+    let mtime = at.metadata(file).modified().unwrap();
+    let mtime_nanos = mtime
+        .duration_since(std::time::UNIX_EPOCH)
+        .unwrap()
+        .as_nanos()
+        .to_string();
+    assert_eq!(result.stdout_str().trim(), &mtime_nanos[..]);
+}
+
 #[test]
 #[cfg(all(unix, not(target_os = "macos")))]
 /// TODO: expected to fail currently; change to `succeeds()` when required.