diff --git a/src/index.js b/src/index.js
index 94ed3b4..5f064f9 100644
--- a/src/index.js
+++ b/src/index.js
@@ -185,7 +185,19 @@ export function projectSchedule(schedule) {
   // Periods. The uncapped counts keep the drift signal alive after the
   // schedule ends; the capped ones are what you would print next to "of N".
   const periodsElapsedUncapped = Math.floor(blocksSoFar / periodBlocks);
-  const periodsExpectedUncapped = Math.floor(secondsSoFar / intendedPeriodSeconds);
+  let periodsExpectedUncapped = Math.floor(secondsSoFar / intendedPeriodSeconds);
+  // A rounded period duration can put the quotient on either side of a boundary.
+  // Check adjacent deadlines using the same start + total blocks * rate
+  // calculation as intendedEnd, so unfinished periods are not counted.
+  const nextPeriodEnd = startTimestamp
+    + (periodsExpectedUncapped + 1) * periodBlocks * designSecondsPerBlock;
+  const currentPeriodEnd = startTimestamp
+    + periodsExpectedUncapped * periodBlocks * designSecondsPerBlock;
+  if (currentTimestamp >= nextPeriodEnd) {
+    periodsExpectedUncapped += 1;
+  } else if (currentTimestamp < currentPeriodEnd) {
+    periodsExpectedUncapped -= 1;
+  }
   const periodsElapsed = Math.min(periods, periodsElapsedUncapped);
   const periodsExpectedByNow = Math.min(periods, periodsExpectedUncapped);
 
diff --git a/test/design-clock-boundaries.test.js b/test/design-clock-boundaries.test.js
new file mode 100644
index 0000000..3bb3ea5
--- /dev/null
+++ b/test/design-clock-boundaries.test.js
@@ -0,0 +1,113 @@
+// SPDX-FileCopyrightText: 2026 1932819331hy-debug
+// SPDX-License-Identifier: MIT
+
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { projectSchedule, checkSchedule } from '../src/index.js';
+
+const startTimestamp = 1_700_000_000;
+const startBlock = 1_000_000;
+const schedule = {
+  startBlock, startTimestamp, periodBlocks: 1003, periods: 10,
+  designSecondsPerBlock: 0.2, observedSecondsPerBlock: 0.2,
+  currentBlock: startBlock + 5015, currentTimestamp: startTimestamp + 1003,
+};
+
+// The immediately preceding representable timestamp, not an arbitrary epsilon.
+function previousNumber(value) {
+  const view = new DataView(new ArrayBuffer(8));
+  view.setFloat64(0, value);
+  view.setBigUint64(0, view.getBigUint64(0) - 1n);
+  return view.getFloat64(0);
+}
+
+for (const { rate, blocks, seconds } of [
+  { rate: 0.2, blocks: 1003, seconds: 1003 }, // 5015 blocks at 200 ms
+  { rate: 0.45, blocks: 2628, seconds: 5913 }, // 13140 blocks at 450 ms
+  { rate: 1.1, blocks: 100, seconds: 550 }, // 500 blocks at 1100 ms
+]) {
+  test(`counts five periods exactly on the ${rate}s design-clock boundary`, () => {
+    const r = projectSchedule({ ...schedule, periodBlocks: blocks,
+      designSecondsPerBlock: rate, observedSecondsPerBlock: rate,
+      currentBlock: startBlock + blocks * 5, currentTimestamp: startTimestamp + seconds });
+    assert.equal(r.periodsExpectedByNow, 5);
+    assert.equal(r.periodsExpectedUncapped, 5);
+    assert.equal(r.periodsElapsed, 5);
+    assert.equal(r.periodsAhead, 0);
+    assert.equal(r.pace, 'on plan');
+    assert.equal(r.driftPercent, 0);
+  });
+
+  test(`does not count the ${rate}s boundary before its timestamp`, () => {
+    const deadline = startTimestamp + seconds;
+    for (const currentTimestamp of [deadline - 1, previousNumber(deadline)]) {
+      const r = projectSchedule({ ...schedule, periodBlocks: blocks,
+        designSecondsPerBlock: rate, currentTimestamp });
+      assert.equal(r.periodsExpectedUncapped, 4);
+      assert.equal(r.periodsExpectedByNow, 4);
+    }
+  });
+}
+
+test('fractional UNIX timestamps obey the same deadline without early rounding', () => {
+  const start = startTimestamp + 0.25;
+  const deadline = start + 1003;
+  for (const [currentTimestamp, expected] of [[previousNumber(deadline), 4], [deadline, 5]]) {
+    const r = projectSchedule({ ...schedule, startTimestamp: start, currentTimestamp });
+    assert.equal(r.periodsExpectedUncapped, expected);
+  }
+});
+
+test('a genesis-origin clock uses total blocks at the five-period deadline', () => {
+  for (const [currentTimestamp, expected] of [[previousNumber(1003), 4], [1003, 5]]) {
+    const r = projectSchedule({ ...schedule, startBlock: 0, currentBlock: 5015,
+      startTimestamp: 0, currentTimestamp });
+    assert.equal(r.periodsExpectedUncapped, expected);
+  }
+});
+
+test('uncapped design counts survive completion and remain independent of actual pace', () => {
+  const complete = projectSchedule({ ...schedule, periods: 3 });
+  assert.equal(complete.periodsExpectedByNow, 3);
+  assert.equal(complete.periodsExpectedUncapped, 5);
+  assert.equal(complete.periodsAhead, 0);
+  assert.equal(complete.complete, true);
+
+  const slower = projectSchedule({ ...schedule, currentTimestamp: startTimestamp + 2006,
+    observedSecondsPerBlock: 0.4 });
+  assert.equal(slower.periodsExpectedByNow, 10);
+  assert.equal(slower.periodsElapsed, 5);
+  assert.equal(slower.periodsAhead, -5);
+  assert.equal(slower.driftPercent, -100);
+  assert.equal(slower.pace, 'slower');
+});
+
+test('checkSchedule preserves expected counts through actual block-header parsing', async () => {
+  const url = 'https://schedule-fixture.invalid';
+  const headers = new Map([
+    ['latest', { number: startBlock + 5015, timestamp: startTimestamp + 1003 }],
+    ['0xf4240', { number: startBlock, timestamp: startTimestamp }],
+  ]);
+  const requested = [];
+  const rpc = { retries: 0, fetchImpl: async (requestUrl, init) => {
+    assert.equal(requestUrl, url);
+    const { method, params: [tag, full] } = JSON.parse(init.body);
+    assert.equal(method, 'eth_getBlockByNumber');
+    assert.equal(full, false);
+    requested.push(tag);
+    const header = headers.get(tag);
+    assert.ok(header, `unknown fixture tag ${tag}`);
+    return new Response(JSON.stringify({ jsonrpc: '2.0', id: 1, result: {
+      number: '0x' + header.number.toString(16), timestamp: '0x' + header.timestamp.toString(16),
+    } }));
+  } };
+  const r = await checkSchedule({ rpcUrl: url, rpc, startBlock, periodBlocks: 1003,
+    periods: 10, designSecondsPerBlock: 0.2, sampleBlocks: 5015 });
+  assert.deepEqual(requested, ['latest', '0xf4240', '0xf4240']);
+  assert.equal(r.observedSecondsPerBlock, 0.2);
+  assert.equal(r.driftPercent, 0);
+  assert.equal(r.pace, 'on plan');
+  assert.equal(r.periodsExpectedByNow, 5);
+  assert.equal(r.periodsElapsed, 5);
+  assert.equal(r.periodsAhead, 0);
+});
diff --git a/test/design-clock-early-counts.test.js b/test/design-clock-early-counts.test.js
new file mode 100644
index 0000000..6d28d03
--- /dev/null
+++ b/test/design-clock-early-counts.test.js
@@ -0,0 +1,68 @@
+// SPDX-FileCopyrightText: 2026 1932819331hy-debug
+// SPDX-License-Identifier: MIT
+
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { projectSchedule } from '../src/index.js';
+
+function adjacentPositiveNumber(value, direction) {
+  const view = new DataView(new ArrayBuffer(8));
+  view.setFloat64(0, value);
+  view.setBigUint64(0, view.getBigUint64(0) + BigInt(direction));
+  return view.getFloat64(0);
+}
+
+const schedule = {
+  startBlock: 1_000_000, currentBlock: 1_000_300,
+  periodBlocks: 3, periods: 100,
+  designSecondsPerBlock: 0.15, observedSecondsPerBlock: 0.15,
+};
+
+// 300 blocks at 150 ms take exactly 45 seconds. Each literal deadline below
+// also equals the public report's intendedEnd; no new rounding rule is used.
+for (const [startTimestamp, deadline] of [[0, 45], [0.25, 45.25], [1, 46]]) {
+  for (const [label, currentTimestamp, expected] of [
+    ['before', adjacentPositiveNumber(deadline, -1), 99],
+    ['at', deadline, 100],
+    ['after', adjacentPositiveNumber(deadline, 1), 100],
+  ]) {
+    test(`${label} the ${deadline}s deadline from a ${startTimestamp}s origin`, () => {
+      const r = projectSchedule({ ...schedule, startTimestamp, currentTimestamp });
+      assert.equal(r.intendedEnd, deadline);
+      assert.equal(r.periodsElapsedUncapped, 100);
+      assert.equal(r.periodsExpectedUncapped, expected);
+      assert.equal(r.periodsExpectedByNow, expected);
+      assert.equal(r.periodsAhead, 100 - expected);
+      assert.equal(r.pace, 'on plan');
+      assert.equal(r.driftPercent, 0);
+      assert.equal(r.complete, true);
+    });
+  }
+}
+
+test('an unfinished design period remains excluded after the block schedule completes', () => {
+  const r = projectSchedule({ ...schedule, periods: 50, startTimestamp: 1,
+    currentTimestamp: adjacentPositiveNumber(46, -1) });
+  assert.equal(r.periodsExpectedUncapped, 99);
+  assert.equal(r.periodsExpectedByNow, 50);
+  assert.equal(r.periodsElapsed, 50);
+  assert.equal(r.periodsAhead, 1);
+  assert.equal(r.complete, true);
+});
+
+test('zero elapsed time never produces a negative design count', () => {
+  for (const startTimestamp of [0, 0.25, 1, 1_700_000_000]) {
+    const r = projectSchedule({ ...schedule, startTimestamp,
+      currentTimestamp: startTimestamp, currentBlock: schedule.startBlock });
+    assert.equal(r.periodsExpectedUncapped, 0);
+    assert.equal(r.periodsExpectedByNow, 0);
+    assert.equal(r.periodsAhead, 0);
+    assert.equal(r.realisedSecondsPerBlock, null);
+    assert.equal(r.complete, false);
+  }
+});
+
+test('the timestamp before the start is still rejected', () => {
+  assert.throws(() => projectSchedule({ ...schedule, startTimestamp: 1,
+    currentTimestamp: adjacentPositiveNumber(1, -1) }), /before startTimestamp/);
+});