diff --git a/__tests__/github-builder/source-date-epoch.test.ts b/__tests__/github-builder/source-date-epoch.test.ts
new file mode 100644
index 00000000..b4abcf07
--- /dev/null
+++ b/__tests__/github-builder/source-date-epoch.test.ts
@@ -0,0 +1,74 @@
+/**
+ * Copyright 2026 actions-toolkit authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {beforeEach, describe, expect, it, vi} from 'vitest';
+import * as github from '@actions/github';
+
+import {SourceDateEpoch} from '../../src/github-builder/source-date-epoch.js';
+
+vi.mock('@actions/github', () => ({
+  context: {
+    repo: {owner: 'example', repo: 'app'},
+    sha: 'event-merge-sha',
+    ref: 'refs/pull/12/merge',
+    payload: {pull_request: {head: {sha: 'head-sha'}}}
+  },
+  getOctokit: vi.fn()
+}));
+
+describe('SourceDateEpoch.resolve', () => {
+  const getCommit = vi.fn();
+
+  beforeEach(() => {
+    getCommit.mockReset();
+    vi.mocked(github.getOctokit).mockReturnValue({rest: {repos: {getCommit}}} as unknown as ReturnType<typeof github.getOctokit>);
+  });
+
+  it.each([
+    {input: '0', epoch: '0', created: '1970-01-01T00:00:00.000Z'},
+    {input: '000123', epoch: '123', created: '1970-01-01T00:02:03.000Z'},
+    {input: '1704067200', epoch: '1704067200', created: '2024-01-01T00:00:00.000Z'},
+    {input: ' 1704067200\n', epoch: '1704067200', created: '2024-01-01T00:00:00.000Z'},
+    {input: '253402300799', epoch: '253402300799', created: '9999-12-31T23:59:59.000Z'}
+  ])('resolves $input without GitHub access', async ({input, epoch, created}) => {
+    await expect(SourceDateEpoch.resolve(input)).resolves.toEqual({epoch, created});
+    expect(github.getOctokit).not.toHaveBeenCalled();
+  });
+
+  it.each(['', ' ', '-1', '+1', '1.5', '1e3', '0x10', 'NaN', 'Infinity', 'context', '253402300800', '8640000000001', '9007199254740993', '1\nOUTPUT=bad'])('rejects invalid input %j without GitHub access', async input => {
+    await expect(SourceDateEpoch.resolve(input)).rejects.toThrow('source-date-epoch must be commit or a non-negative Unix timestamp through year 9999');
+    expect(github.getOctokit).not.toHaveBeenCalled();
+  });
+
+  it('uses the event SHA and committer time, not the PR head or author time', async () => {
+    getCommit.mockResolvedValue({data: {commit: {committer: {date: '2024-01-01T00:00:00Z'}, author: {date: '2000-01-01T00:00:00Z'}}}});
+
+    await expect(SourceDateEpoch.resolve('commit', {token: 'custom-token'})).resolves.toEqual({epoch: '1704067200', created: '2024-01-01T00:00:00.000Z'});
+    expect(github.getOctokit).toHaveBeenCalledWith('custom-token');
+    expect(getCommit).toHaveBeenCalledExactlyOnceWith({owner: 'example', repo: 'app', ref: 'event-merge-sha'});
+  });
+
+  it.each([null, {}, {date: 'invalid'}, {date: '1969-12-31T23:59:59Z'}])('rejects an unusable commit timestamp %j', async committer => {
+    getCommit.mockResolvedValue({data: {commit: {committer}}});
+    await expect(SourceDateEpoch.resolve('commit')).rejects.toThrow('source-date-epoch must be commit or a non-negative Unix timestamp through year 9999');
+  });
+
+  it('propagates GitHub errors instead of substituting the current time', async () => {
+    const error = new Error('Commit not found');
+    getCommit.mockRejectedValue(error);
+    await expect(SourceDateEpoch.resolve('commit')).rejects.toBe(error);
+  });
+});
diff --git a/src/github-builder/source-date-epoch.ts b/src/github-builder/source-date-epoch.ts
new file mode 100644
index 00000000..82cf0bee
--- /dev/null
+++ b/src/github-builder/source-date-epoch.ts
@@ -0,0 +1,42 @@
+/**
+ * Copyright 2026 actions-toolkit authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {GitHub, GitHubOpts} from '../github/github.js';
+
+export class SourceDateEpoch {
+  /**
+   * Resolves a non-negative Unix timestamp through year 9999, or the event
+   * commit's committer time for "commit". Empty input is invalid; callers should
+   * skip resolution when reproducible timestamps are disabled.
+   */
+  public static async resolve(input: string, opts?: GitHubOpts): Promise<{epoch: string; created: string}> {
+    let value = input.trim();
+    if (value === 'commit') {
+      const {data} = await new GitHub(opts).octokit.rest.repos.getCommit({
+        ...GitHub.context.repo,
+        ref: GitHub.context.sha
+      });
+      value = String(Date.parse(data.commit.committer?.date || '') / 1000);
+    }
+
+    const epoch = Number(value);
+    const date = new Date(epoch * 1000);
+    if (!/^\d+$/.test(value) || !Number.isSafeInteger(epoch) || Number.isNaN(date.getTime()) || date.getUTCFullYear() > 9999) {
+      throw new Error('source-date-epoch must be commit or a non-negative Unix timestamp through year 9999');
+    }
+    return {epoch: String(epoch), created: date.toISOString()};
+  }
+}