diff --git a/crates/series/src/main.rs b/crates/series/src/main.rs
index 2c32c09..f8d918b 100644
--- a/crates/series/src/main.rs
+++ b/crates/series/src/main.rs
@@ -89,7 +89,7 @@ fn main() -> ExitCode {
         return ExitCode::FAILURE;
     }
 
-    points.sort_by_key(|(ts, _)| ts.timestamp());
+    points.sort_by_key(|(ts, _)| *ts);
     render(&points);
     ExitCode::SUCCESS
 }
diff --git a/crates/series/tests/cli.rs b/crates/series/tests/cli.rs
index a3f65b7..10f1a96 100644
--- a/crates/series/tests/cli.rs
+++ b/crates/series/tests/cli.rs
@@ -34,6 +34,20 @@ fn sorts_and_summarizes() {
     assert!(stdout.contains('▁'), "no trough block:\n{stdout}");
 }
 
+#[test]
+fn sorts_within_the_same_second() {
+    // rows print at second resolution, so the values tell the order apart
+    let out = run("2026-09-03T10:20:00.900+09:00 9\n2026-09-03T10:20:00.100+09:00 1\n");
+    assert!(out.status.success());
+    let stdout = String::from_utf8(out.stdout).unwrap();
+    let values: Vec<&str> = stdout
+        .lines()
+        .take_while(|l| !l.is_empty())
+        .filter_map(|l| l.split_whitespace().last())
+        .collect();
+    assert_eq!(values, ["1", "9"], "stdout:\n{stdout}");
+}
+
 #[test]
 fn skips_garbage_but_still_succeeds() {
     let out = run(include_str!("fixtures/invalid.txt"));