fix(date): align pure-digit -d parsing with GNU semantics

Implement GNU 'Pure numbers in date strings' for time-of-day tokens:
- 1–2 digits => HH:00 today
- 3–4 digits => HHMM today
- Validate ranges; reject invalid times (e.g., 2400, 2360)

Also:
- Add tests for -d0, -d7, -d0700 under TZ=UTC0 and invalid numeric inputs
- Reference GNU manual section for pure numbers

src/uu/date/src/date.rs

@@ -203,7 +203,48 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     // Iterate over all dates - whether it's a single date or a file.
     let dates: Box<dyn Iterator<Item = _>> = match settings.date_source {
         DateSource::Human(ref input) => {
-            let date = parse_date(input);
+            // GNU compatibility (Pure numbers in date strings):
+            // - Manual: https://www.gnu.org/software/coreutils/manual/html_node/Pure-numbers-in-date-strings.html
+            // - Semantics: a pure decimal number denotes today’s time-of-day (HH or HHMM).
+            //   Examples: "0"/"00" => 00:00 today; "7"/"07" => 07:00 today; "0700" => 07:00 today.
+            // For all other forms, fall back to the general parser.
+            let is_pure_digits =
+                !input.is_empty() && input.len() <= 4 && input.chars().all(|c| c.is_ascii_digit());
+
+            let date = if is_pure_digits {
+                // Derive HH and MM from the input
+                let (hh_opt, mm_opt) = if input.len() <= 2 {
+                    (input.parse::<u32>().ok(), Some(0u32))
+                } else {
+                    let (h, m) = input.split_at(input.len() - 2);
+                    (h.parse::<u32>().ok(), m.parse::<u32>().ok())
+                };
+
+                if let (Some(hh), Some(mm)) = (hh_opt, mm_opt) {
+                    // Compose a concrete datetime string for today with zone offset.
+                    // Use the already-determined 'now' and settings.utc to select offset.
+                    let date_part =
+                        strtime::format("%F", &now).unwrap_or_else(|_| String::from("1970-01-01"));
+                    // If -u, force +00:00; otherwise use the local offset of 'now'.
+                    let offset = if settings.utc {
+                        String::from("+00:00")
+                    } else {
+                        strtime::format("%:z", &now).unwrap_or_default()
+                    };
+                    let composed = if offset.is_empty() {
+                        format!("{date_part} {hh:02}:{mm:02}")
+                    } else {
+                        format!("{date_part} {hh:02}:{mm:02} {offset}")
+                    };
+                    parse_date(composed)
+                } else {
+                    // Fallback on parse failure of digits
+                    parse_date(input)
+                }
+            } else {
+                parse_date(input)
+            };
+
             let iter = std::iter::once(date);
             Box::new(iter)
         }

tests/by-util/test_date.rs

@@ -778,3 +778,60 @@ fn test_date_resolution_no_combine() {
         .arg("2025-01-01")
         .fails();
 }
+
+#[test]
+fn test_date_numeric_d_basic_utc() {
+    // Verify GNU-compatible pure-digit parsing for -d STRING under UTC
+    // 0/00 -> today at 00:00; 7/07 -> today at 07:00; 0700 -> today at 07:00
+    let today = Utc::now().date_naive();
+    let yyyy = today.year();
+    let mm = today.month();
+    let dd = today.day();
+
+    let mk =
+        |h: u32, m: u32| -> String { format!("{yyyy:04}-{mm:02}-{dd:02} {h:02}:{m:02}:00 UTC\n") };
+
+    new_ucmd!()
+        .env("TZ", "UTC0")
+        .arg("-d")
+        .arg("0")
+        .arg("+%F %T %Z")
+        .succeeds()
+        .stdout_only(mk(0, 0));
+
+    new_ucmd!()
+        .env("TZ", "UTC0")
+        .arg("-d")
+        .arg("7")
+        .arg("+%F %T %Z")
+        .succeeds()
+        .stdout_only(mk(7, 0));
+
+    new_ucmd!()
+        .env("TZ", "UTC0")
+        .arg("-d")
+        .arg("0700")
+        .arg("+%F %T %Z")
+        .succeeds()
+        .stdout_only(mk(7, 0));
+}
+
+#[test]
+fn test_date_numeric_d_invalid_numbers() {
+    // Ensure invalid HHMM values are rejected (GNU-compatible)
+    new_ucmd!()
+        .env("TZ", "UTC0")
+        .arg("-d")
+        .arg("2400")
+        .arg("+%F %T %Z")
+        .fails()
+        .stderr_contains("invalid date");
+
+    new_ucmd!()
+        .env("TZ", "UTC0")
+        .arg("-d")
+        .arg("2360")
+        .arg("+%F %T %Z")
+        .fails()
+        .stderr_contains("invalid date");
+}