diff --git a/tests/fix-2136-clock-local-today.test.cjs b/tests/fix-2136-clock-local-today.test.cjs
new file mode 100644
index 000000000..d824b229a
--- /dev/null
+++ b/tests/fix-2136-clock-local-today.test.cjs
@@ -0,0 +1,120 @@
+'use strict';
+
+/**
+ * #2136 — operator-facing date-only fields must use the HOST-LOCAL calendar day,
+ * not the UTC calendar day.
+ *
+ * `clock.today()` derived the day by slicing a UTC ISO instant
+ * (`nowIso().split('T')[0]`), so in any negative-UTC-offset zone during UTC's
+ * early hours `last_activity` named a day the operator had not reached yet — a
+ * day AHEAD of `last_updated`'s local date, written by the same call.
+ *
+ * The fix adds `clock.localToday()` (host-local calendar day, honoring the same
+ * GSD_NOW_MS pin) and routes operator-facing date-only fields through it. These
+ * tests prove:
+ *   (1) realClock.localToday() returns the local day under a pinned instant + TZ;
+ *   (2) realClock.today() is unchanged (UTC) — internal/cosmetic stamps stay UTC;
+ *   (3) the fake clock mirrors localToday();
+ *   (4) the `last_activity` field is stamped from localToday, not today
+ *       (a split clock where the two differ proves the wiring).
+ */
+
+const { describe, test } = require('node:test');
+const assert = require('node:assert/strict');
+const { execFileSync } = require('node:child_process');
+const path = require('node:path');
+
+const CLOCK_CJS = path.join(__dirname, '..', 'gsd-core', 'bin', 'lib', 'clock.cjs');
+const STATE_TRANSITION_CJS = path.join(__dirname, '..', 'gsd-core', 'bin', 'lib', 'state-transition.cjs');
+const { stateExtractField } = require('../gsd-core/bin/lib/state-document.cjs');
+const { makeFakeClock } = require('./helpers/clock.cjs');
+
+// 2020-06-15T02:00:00.000Z. Under America/Chicago (UTC−5 in June) this instant
+// is 2020-06-14 21:00 local — the local calendar day is 2020-06-14 while the UTC
+// calendar day is 2020-06-15. This is the issue's exact repro instant.
+const PINNED_MS = '1592186400000';
+
+/**
+ * Run a one-liner in a FRESH subprocess so TZ is set at process start (Node
+ * caches timezone; runtime mutation of process.env.TZ is unreliable across
+ * versions). GSD_TEST_MODE + GSD_NOW_MS pin realClock via the documented seam.
+ */
+function clockInSubprocess(expr, env) {
+  return execFileSync(process.execPath, ['-e', expr], {
+    env,
+    encoding: 'utf8',
+  }).trim();
+}
+
+describe('#2136 realClock.localToday() — host-local calendar day', () => {
+  test('returns the LOCAL calendar day under a negative-UTC-offset zone', () => {
+    const out = clockInSubprocess(
+      `const {realClock}=require(${JSON.stringify(CLOCK_CJS)});process.stdout.write(realClock.localToday());`,
+      { PATH: process.env.PATH, GSD_TEST_MODE: '1', GSD_NOW_MS: PINNED_MS, TZ: 'America/Chicago' },
+    );
+    assert.strictEqual(out, '2020-06-14',
+      'localToday() must be the local calendar day (2020-06-14 in Chicago), not the UTC day (2020-06-15)');
+  });
+
+  test('realClock.today() is unchanged (UTC calendar day) — internal stamps stay UTC', () => {
+    const out = clockInSubprocess(
+      `const {realClock}=require(${JSON.stringify(CLOCK_CJS)});process.stdout.write(realClock.today());`,
+      { PATH: process.env.PATH, GSD_TEST_MODE: '1', GSD_NOW_MS: PINNED_MS, TZ: 'America/Chicago' },
+    );
+    assert.strictEqual(out, '2020-06-15',
+      'today() must remain the UTC calendar day; only operator-facing fields moved to localToday()');
+  });
+
+  test('localToday === today on a UTC host (no spurious divergence)', () => {
+    const out = clockInSubprocess(
+      `const {realClock}=require(${JSON.stringify(CLOCK_CJS)});process.stdout.write(realClock.localToday()+'|'+realClock.today());`,
+      { PATH: process.env.PATH, GSD_TEST_MODE: '1', GSD_NOW_MS: PINNED_MS, TZ: 'UTC' },
+    );
+    const [local, utc] = out.split('|');
+    assert.strictEqual(local, '2020-06-15');
+    assert.strictEqual(utc, '2020-06-15');
+  });
+});
+
+describe('#2136 makeFakeClock mirrors localToday()', () => {
+  test('fake clock localToday() derives the local day from the pinned epoch', () => {
+    const clock = makeFakeClock(Number(PINNED_MS));
+    // The fake honors the host TZ of the test process; assert it returns a
+    // YYYY-MM-DD string derived from local Date methods (structure + determinism
+    // under the pinned epoch), mirroring realClock.localToday().
+    assert.ok(/^\d{4}-\d{2}-\d{2}$/.test(clock.localToday()),
+      'fake localToday() must return a YYYY-MM-DD string');
+    assert.strictEqual(typeof clock.localToday, 'function',
+      'makeFakeClock must expose localToday so it stays a drop-in Clock substitute');
+  });
+});
+
+describe('#2136 last_activity is stamped from localToday, not today', () => {
+  // A clock where the UTC day and the local day DIFFER. If the transition core
+  // still consulted today(), Last Activity would read the UTC value; the fix
+  // routes it through localToday().
+  const splitClock = Object.freeze({
+    today: () => '2020-06-15',
+    localToday: () => '2020-06-14',
+    nowIso: () => '2020-06-15T02:00:00.000Z',
+  });
+
+  test('sync transition writes the LOCAL day into Last Activity', () => {
+    const { transitionCore } = require(STATE_TRANSITION_CJS);
+    const input = [
+      '# Project State',
+      '',
+      '**Total Plans in Phase:** 2',
+      '**Last Activity:** 2020-06-10',
+      '**Progress:** [████░░░░░░] 40%',
+      '',
+    ].join('\n');
+    const result = transitionCore(
+      input,
+      { kind: 'sync', totalPlansInPhase: 5, percent: 60 },
+      { clock: splitClock, progressProvider: () => null },
+    );
+    assert.strictEqual(stateExtractField(result.content, 'Last Activity'), '2020-06-14',
+      'Last Activity must use localToday() (2020-06-14), not today() (2020-06-15)');
+  });
+});
diff --git a/tests/helpers/clock.cjs b/tests/helpers/clock.cjs
index 87146c8ea..56a3c93ee 100644
--- a/tests/helpers/clock.cjs
+++ b/tests/helpers/clock.cjs
@@ -90,6 +90,20 @@ function makeFakeClock(startMs) {
       return new Date(_now).toISOString().split('T')[0];
     },
 
+    /**
+     * Return the virtual date as a YYYY-MM-DD string in the HOST-LOCAL calendar
+     * day. Mirrors realClock.localToday() so fake clocks are drop-in substitutes.
+     * (#2136: operator-facing date-only fields use the local calendar day.)
+     *
+     * @returns {string} e.g. "2020-06-14" (local), may differ from today() near UTC midnight
+     */
+    localToday() {
+      const d = new Date(_now);
+      const mm = String(d.getMonth() + 1).padStart(2, '0');
+      const dd = String(d.getDate()).padStart(2, '0');
+      return d.getFullYear() + '-' + mm + '-' + dd;
+    },
+
     /** Array of ms values passed to sleep() in call order. */
     get sleepCalls() {
       return _sleepCalls;
diff --git a/tests/state-rebuild.test.cjs b/tests/state-rebuild.test.cjs
index ba4492e85..d9ebb7b68 100644
--- a/tests/state-rebuild.test.cjs
+++ b/tests/state-rebuild.test.cjs
@@ -25,6 +25,7 @@ const { stateExtractField } = require('../gsd-core/bin/lib/state-document.cjs');
 
 const fixedClock = Object.freeze({
   today: () => '2026-06-29',
+  localToday: () => '2026-06-29',
   nowIso: () => '2026-06-29T12:00:00.000Z',
 });
 
diff --git a/tests/state-transition.test.cjs b/tests/state-transition.test.cjs
index 349d2f703..71f3b090f 100644
--- a/tests/state-transition.test.cjs
+++ b/tests/state-transition.test.cjs
@@ -22,6 +22,7 @@ const { stateExtractField } = require('../gsd-core/bin/lib/state-document.cjs');
 
 const fixedClock = Object.freeze({
   today: () => '2026-06-27',
+  localToday: () => '2026-06-27',
   nowIso: () => '2026-06-27T12:00:00.000Z',
 });