diff --git a/static/js/tasks.js b/static/js/tasks.js
index a4ffc63..73aa39c 100644
--- a/static/js/tasks.js
+++ b/static/js/tasks.js
@@ -7,6 +7,7 @@ import markdownModule from './markdown.js';
 import * as spinnerModule from './spinner.js';
 import { makeWindowDraggable } from './windowDrag.js';
 import { sortModelIds } from './modelSort.js';
+import { ordinalSuffix } from './util/ordinal.js';
 
 const API_BASE = window.location.origin;
 let _open = false;
@@ -244,7 +245,7 @@ function _scheduleLabel(task) {
   }
   if (task.schedule === 'monthly') {
     const d = task.scheduled_day ?? 1;
-    const suffix = d === 1 ? 'st' : d === 2 ? 'nd' : d === 3 ? 'rd' : 'th';
+    const suffix = ordinalSuffix(d);
     return `Monthly on ${d}${suffix} at ${localTime}`;
   }
   return task.schedule || '—';
diff --git a/static/js/util/ordinal.js b/static/js/util/ordinal.js
new file mode 100644
index 0000000..20c37d4
--- /dev/null
+++ b/static/js/util/ordinal.js
@@ -0,0 +1,13 @@
+// Pure (browser-free) English ordinal suffix, e.g. 1 -> "st", 21 -> "st",
+// 22 -> "nd", 23 -> "rd", 11/12/13 -> "th". Extracted so it can be unit-tested.
+export function ordinalSuffix(n) {
+  const a = Math.abs(Math.trunc(Number(n) || 0));
+  const mod100 = a % 100;
+  if (mod100 >= 11 && mod100 <= 13) return 'th';
+  switch (a % 10) {
+    case 1: return 'st';
+    case 2: return 'nd';
+    case 3: return 'rd';
+    default: return 'th';
+  }
+}
diff --git a/tests/test_ordinal_suffix_js.py b/tests/test_ordinal_suffix_js.py
new file mode 100644
index 0000000..54f90f4
--- /dev/null
+++ b/tests/test_ordinal_suffix_js.py
@@ -0,0 +1,35 @@
+"""Pin the ordinal-suffix helper used by the monthly-schedule label in tasks.js.
+
+_scheduleLabel built the suffix with `d === 1 ? 'st' : d === 2 ? 'nd' : ...`,
+which only handles single digits, so a monthly task on day 21/22/23/31 rendered
+"Monthly on 21th"/"22th"/"23th"/"31th". The shared ordinalSuffix() fixes this.
+"""
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parent.parent
+_HELPER = _REPO / "static" / "js" / "util" / "ordinal.js"
+_HAS_NODE = shutil.which("node") is not None
+
+
+def _suffixes(nums):
+    arr = json.dumps(nums)
+    js = f"""
+    import {{ ordinalSuffix }} from '{_HELPER.as_posix()}';
+    console.log(JSON.stringify({arr}.map(n => n + ordinalSuffix(n))));
+    """
+    proc = subprocess.run(["node", "--input-type=module"], input=js,
+                          capture_output=True, text=True, cwd=str(_REPO), timeout=30)
+    assert proc.returncode == 0, proc.stderr
+    return json.loads(proc.stdout.strip())
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_ordinal_suffixes_for_days_of_month():
+    assert _suffixes([1, 2, 3, 4, 11, 12, 13, 21, 22, 23, 31]) == [
+        "1st", "2nd", "3rd", "4th", "11th", "12th", "13th", "21st", "22nd", "23rd", "31st",
+    ]