last: show the first record's time in "begins" when -n, --since or --until skip records

src/uu/last/src/platform/unix.rs

@@ -222,23 +222,22 @@ impl Last {
             ut_stack.push(*utmpx);
         });
 
+        // "begins" is the time of the first record, whatever -n, --since and
+        // --until skip
+        let first_ut_time = ut_stack.first().map(|ut| {
+            let first_login_time = ut.login_time();
+            self.utmp_file_time(
+                first_login_time.unix_timestamp(),
+                first_login_time.nanosecond().into(),
+            )
+        });
+
         let mut counter = 0;
-        let mut first_ut_time = None;
         while let Some(ut) = ut_stack.pop() {
             if ut.login_time() < self.since || ut.login_time() > self.until {
                 continue;
             }
 
-            if ut_stack.is_empty() {
-                // By the end of loop we will have the earliest time
-                // (This avoids getting into issues with the compiler)
-                let first_login_time = ut.login_time();
-                first_ut_time = Some(self.utmp_file_time(
-                    first_login_time.unix_timestamp(),
-                    first_login_time.nanosecond().into(),
-                ));
-            }
-
             if counter >= self.limit && self.limit > 0 {
                 break;
             }

tests/by-util/test_last.rs

@@ -144,14 +144,17 @@ fn test_since_only_shows_entries_after_time() {
     let expected_entry_time = "16:29";
     let unexpected_entry_time = "16:24";
 
-    new_ucmd!()
+    let result = new_ucmd!()
         .arg("--file")
         .arg("last.input.1")
         .arg("--since")
         .arg("2025-03-08 16:28")
-        .succeeds()
-        .stdout_contains(expected_entry_time)
-        .stdout_does_not_contain(unexpected_entry_time);
+        .succeeds();
+
+    // The "begins" line after the entries shows the first record, 16:24:36
+    let entries = result.stdout_str().split("\n\n").next().unwrap();
+    assert!(entries.contains(expected_entry_time));
+    assert!(!entries.contains(unexpected_entry_time));
 }
 
 #[test]
@@ -169,3 +172,17 @@ fn test_until_only_shows_entries_before_time() {
         .stdout_contains(expected_entry_time)
         .stdout_does_not_contain(unexpected_entry_time);
 }
+
+#[test]
+#[cfg(all(unix, not(target_os = "macos"), not(target_os = "openbsd")))]
+fn test_begins_is_the_first_record_when_records_are_skipped() {
+    for args in [&["-n", "1"][..], &["--since", "2025-03-08 16:28"][..]] {
+        new_ucmd!()
+            .env("TZ", "UTC")
+            .arg("--file")
+            .arg("last.input.1")
+            .args(args)
+            .succeeds()
+            .stdout_contains("last.input.1 begins Sat Mar  8 16:24:36 2025");
+    }
+}