diff --git a/src/pure.js b/src/pure.js
index 9fd74e6..6f610aa 100644
--- a/src/pure.js
+++ b/src/pure.js
@@ -50,3 +50,10 @@ export function computeEstimate(current, target, perTrain, perDay) {
 
   return { remaining, trainsToGo, days, eta };
 }
+
+const THIRTY_DAYS_MS = 30 * MS_PER_DAY;
+
+export function pruneHistory(entries, now = Date.now()) {
+  const cutoff = now - THIRTY_DAYS_MS;
+  return entries.filter((e) => e.ts > cutoff);
+}
diff --git a/tests/pure.test.js b/tests/pure.test.js
index 3b14f78..76f5cb2 100644
--- a/tests/pure.test.js
+++ b/tests/pure.test.js
@@ -1,6 +1,9 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { parseTarget, computeEstimate } from '../src/pure.js';
+import { parseTarget, computeEstimate, pruneHistory } from '../src/pure.js';
+
+const DAY = 86_400_000;
+const NOW = 1_700_000_000_000; // fixed reference
 
 test('parseTarget: integer numbers', () => {
   assert.equal(parseTarget(25), 25);
@@ -67,3 +70,27 @@ test('computeEstimate: zero perTrain or perDay does not crash', () => {
   assert.equal(b.days, 0);
   assert.equal(b.eta, null);
 });
+
+test('pruneHistory: keeps entries within 30 days, drops older', () => {
+  const entries = [
+    { ts: NOW, delta: 100 },
+    { ts: NOW - 1 * DAY, delta: 100 },
+    { ts: NOW - 29 * DAY, delta: 100 },
+    { ts: NOW - 30 * DAY, delta: 100 }, // exactly 30 days, dropped (strict <)
+    { ts: NOW - 31 * DAY, delta: 100 },
+  ];
+  const out = pruneHistory(entries, NOW);
+  assert.equal(out.length, 3);
+  assert.deepEqual(out.map((e) => e.ts), [NOW, NOW - 1 * DAY, NOW - 29 * DAY]);
+});
+
+test('pruneHistory: does not mutate input', () => {
+  const entries = [{ ts: NOW - 31 * DAY, delta: 100 }];
+  const copy = [...entries];
+  pruneHistory(entries, NOW);
+  assert.deepEqual(entries, copy);
+});
+
+test('pruneHistory: empty input', () => {
+  assert.deepEqual(pruneHistory([], NOW), []);
+});