diff --git a/.github/workflows/unit-tests.yml b/.github/workflows/unit-tests.yml
index 4f46823..78c06c3 100644
--- a/.github/workflows/unit-tests.yml
+++ b/.github/workflows/unit-tests.yml
@@ -26,6 +26,9 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Run type check
+        run: npm run typecheck
+
       - name: Run tests with coverage
         run: npm run test:ci
 
diff --git a/death-clock-core.js b/death-clock-core.js
index 7515dfb..c31d0f3 100644
--- a/death-clock-core.js
+++ b/death-clock-core.js
@@ -184,9 +184,9 @@ function formatTokenCountShort(n) {
 
 /**
  * Return all milestones whose token threshold has been reached.
- * @param {number} tokens - current cumulative token count
- * @param {Array}  milestones
- * @returns {Array}
+ * @param {number}     tokens - current cumulative token count
+ * @param {Milestone[]} milestones
+ * @returns {Milestone[]}
  */
 function getTriggeredMilestones(tokens, milestones) {
   if (typeof tokens !== 'number' || !Array.isArray(milestones)) return [];
@@ -195,9 +195,9 @@ function getTriggeredMilestones(tokens, milestones) {
 
 /**
  * Return the next milestone not yet reached.
- * @param {number} tokens
- * @param {Array}  milestones
- * @returns {Object|null}
+ * @param {number}      tokens
+ * @param {Milestone[]} milestones
+ * @returns {Milestone|null}
  */
 function getNextMilestone(tokens, milestones) {
   if (typeof tokens !== 'number' || !Array.isArray(milestones)) return null;
@@ -278,7 +278,7 @@ function generateProjectionData(currentTokens, ratePerSec, months, now, annualGr
   for (let i = 0; i <= months; i++) {
     const d = new Date(base.getTime());
     d.setMonth(d.getMonth() + i);
-    const elapsed = (d - base) / 1000; // seconds since base
+    const elapsed = (d.getTime() - base.getTime()) / 1000; // seconds since base
     let additionalTokens;
     if (growth === 0) {
       additionalTokens = ratePerSec * elapsed;
@@ -316,7 +316,7 @@ function formatDate(date) {
 function getTimeDelta(date, now) {
   if (!(date instanceof Date) || isNaN(date.getTime())) return '';
   const base = now instanceof Date ? now : new Date();
-  const diff = date - base;
+  const diff = date.getTime() - base.getTime();
   if (diff <= 0) return 'Already passed';
   const totalSeconds = Math.floor(diff / 1000);
   const totalMinutes = Math.floor(diff / (1000 * 60));
@@ -564,8 +564,8 @@ const COMPANY_STAGES = [
  * Compute the total passive token generation rate (tokens/sec) from owned AI
  * agents and fired (replaced) company roles.
  *
- * @param {Object} ownedAgents   - { agentId: count }  (non-integer counts are floored)
- * @param {Object} replacedRoles - { roleId: true }
+ * @param {Object.<string, number>}  ownedAgents   - { agentId: count }  (non-integer counts are floored)
+ * @param {Object.<string, boolean>} replacedRoles - { roleId: true }
  * @returns {number}               tokens per second
  */
 function computePassiveRate(ownedAgents, replacedRoles) {
@@ -616,9 +616,9 @@ const SESSION_CHALLENGE_DEFS = [
 
 /**
  * Return the first personal milestone that the player has not yet crossed.
- * @param {number} personalTokens
- * @param {Array}  milestones
- * @returns {Object|null}
+ * @param {number}      personalTokens
+ * @param {Milestone[]} milestones
+ * @returns {Milestone|null}
  */
 function getNextMilestoneForPlayer(personalTokens, milestones) {
   if (typeof personalTokens !== 'number' || !Array.isArray(milestones)) return null;
diff --git a/global.d.ts b/global.d.ts
new file mode 100644
index 0000000..ca3329e
--- /dev/null
+++ b/global.d.ts
@@ -0,0 +1,46 @@
+// ============================================================
+// Global type extensions for the browser dual-export pattern.
+//
+// death-clock-core.js, milestones-data.js, changelog-data.js,
+// and project-stats-data.js each assign a property on `window`
+// when loaded as classic <script> tags.  TypeScript needs these
+// declarations to type-check the window-branch of the dual-export
+// pattern in death-clock-core.js without errors.
+// ============================================================
+
+/** A single environmental-milestone entry from milestones.yaml */
+interface Milestone {
+  id: string;
+  name: string;
+  icon: string;
+  tokens: number;
+  shortDesc: string;
+  description: string;
+  consequence: string;
+  followingEvent: string;
+  color: string;
+  darkColor: string;
+  reference?: string;
+  extinctionMarker?: boolean;
+}
+
+interface ChangelogSection {
+  heading: string;
+  items: string[];
+}
+
+interface ChangelogRelease {
+  version: string;
+  date: string | null;
+  sections: ChangelogSection[];
+}
+
+// Extend the standard Window interface with the properties set by the
+// auto-generated data modules and the core module itself.
+interface Window {
+  MilestonesData?: { MILESTONES: Milestone[] };
+  /** The core module's exported API surface (plain-object namespace). */
+  DeathClockCore?: Record<string, unknown>;
+  ChangelogData?: { SITE_VERSION: string; CHANGELOG_RELEASES: ChangelogRelease[] };
+  ProjectStatsData?: { PROJECT_PR_COUNT: number; PROJECT_TOTAL_TOKENS: number };
+}
diff --git a/package-lock.json b/package-lock.json
index 8c4a818..be392c1 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -9,10 +9,12 @@
       "version": "1.3.0",
       "devDependencies": {
         "@playwright/test": "^1.59.1",
+        "@types/js-yaml": "^4.0.9",
         "jest": "^30.3.0",
         "jest-environment-jsdom": "^30.3.0",
         "js-yaml": "^4.1.0",
-        "serve": "^14.2.6"
+        "serve": "^14.2.6",
+        "typescript": "^6.0.3"
       }
     },
     "node_modules/@asamuzakjp/css-color": {
@@ -1385,6 +1387,13 @@
         "@types/istanbul-lib-report": "*"
       }
     },
+    "node_modules/@types/js-yaml": {
+      "version": "4.0.9",
+      "resolved": "https://registry.npmjs.org/@types/js-yaml/-/js-yaml-4.0.9.tgz",
+      "integrity": "sha512-k4MGaQl5TGo/iipqb2UDG2UwjXziSWkh0uysQelTlJpX1qGlpUZYm8PnO4DxG1qBomtJUdYJ6qR6xdIah10JLg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/jsdom": {
       "version": "21.1.7",
       "resolved": "https://registry.npmjs.org/@types/jsdom/-/jsdom-21.1.7.tgz",
@@ -5125,6 +5134,20 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/typescript": {
+      "version": "6.0.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-6.0.3.tgz",
+      "integrity": "sha512-y2TvuxSZPDyQakkFRPZHKFm+KKVqIisdg9/CZwm9ftvKXLP8NRWj38/ODjNbr43SsoXqNuAisEf1GdCxqWcdBw==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
+    },
     "node_modules/undici-types": {
       "version": "7.19.2",
       "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.19.2.tgz",
diff --git a/package.json b/package.json
index f5faa68..7d1b74a 100644
--- a/package.json
+++ b/package.json
@@ -4,6 +4,7 @@
   "description": "AI Death Clock — visualising the environmental cost of global AI token consumption",
   "homepage": "https://nitrocode.github.io/token-deathclock/",
   "scripts": {
+    "typecheck": "tsc --noEmit",
     "test": "jest --coverage",
     "test:ci": "jest --ci --coverage",
     "test:e2e": "playwright test",
@@ -14,10 +15,12 @@
   },
   "devDependencies": {
     "@playwright/test": "^1.59.1",
+    "@types/js-yaml": "^4.0.9",
     "jest": "^30.3.0",
     "jest-environment-jsdom": "^30.3.0",
     "js-yaml": "^4.1.0",
-    "serve": "^14.2.6"
+    "serve": "^14.2.6",
+    "typescript": "^6.0.3"
   },
   "jest": {
     "testEnvironment": "node",
diff --git a/project-stats-data.js b/project-stats-data.js
index e703756..b4b6253 100644
--- a/project-stats-data.js
+++ b/project-stats-data.js
@@ -2,8 +2,8 @@
 // AUTO-GENERATED from project-stats.yaml — do not edit directly.
 // Run `npm run build:project-stats` to regenerate from project-stats.yaml.
 
-const PROJECT_PR_COUNT     = 47;
-const PROJECT_TOTAL_TOKENS = 7000000;
+const PROJECT_PR_COUNT     = 48;
+const PROJECT_TOTAL_TOKENS = 7200000;
 
 /* istanbul ignore next */
 if (typeof module !== 'undefined' && module.exports) {
diff --git a/project-stats.yaml b/project-stats.yaml
index 72900fa..83173df 100644
--- a/project-stats.yaml
+++ b/project-stats.yaml
@@ -8,5 +8,5 @@
 # Run `npm run build:project-stats` (or let the deploy workflow do it) to
 # regenerate project-stats-data.js from this file.
 
-pr_count: 47
-total_tokens: 7000000
+pr_count: 48
+total_tokens: 7200000
diff --git a/scripts/build-changelog.js b/scripts/build-changelog.js
index b58f863..f9d1b12 100644
--- a/scripts/build-changelog.js
+++ b/scripts/build-changelog.js
@@ -28,8 +28,14 @@ const SITE_VERSION = pkg.version || '0.0.0';
 // ── Read and parse CHANGELOG.md ──────────────────────────────────────────────
 const raw = fs.readFileSync(SRC, 'utf8');
 
+/** @typedef {{ heading: string, items: string[] }} Section */
+/** @typedef {{ version: string, date: string|null, sections: Section[] }} Release */
+
+/** @type {Release[]} */
 const releases = [];
+/** @type {Release|null} */
 let currentRelease = null;
+/** @type {Section|null} */
 let currentSection = null;
 
 for (const line of raw.split('\n')) {
@@ -71,15 +77,18 @@ if (releases.length === 0) {
 }
 
 // ── Generate JS ───────────────────────────────────────────────────────────────
+/** @param {string} s */
 function jsStr(s) {
   return JSON.stringify(String(s));
 }
 
+/** @param {Section} sec */
 function renderSection(sec) {
   const itemLines = sec.items.map((item) => `        ${jsStr(item)},`).join('\n');
   return `      { heading: ${jsStr(sec.heading)}, items: [\n${itemLines}\n      ] }`;
 }
 
+/** @param {Release} r */
 function renderRelease(r) {
   const secLines = r.sections.map(renderSection).join(',\n');
   const datePart = r.date ? jsStr(r.date) : 'null';
diff --git a/scripts/build-milestones.js b/scripts/build-milestones.js
index 26c1506..d9c902d 100644
--- a/scripts/build-milestones.js
+++ b/scripts/build-milestones.js
@@ -27,7 +27,18 @@ const REQUIRED_FIELDS = [
 
 // ── Load & parse ─────────────────────────────────────────────────────────────
 const raw = fs.readFileSync(SRC, 'utf8');
-const doc = yaml.load(raw);
+
+/**
+ * @typedef {{
+ *   id: string, name: string, icon: string, tokens: number,
+ *   shortDesc: string, description: string, consequence: string,
+ *   followingEvent: string, color: string, darkColor: string,
+ *   reference?: string, extinctionMarker?: boolean,
+ *   [key: string]: unknown
+ * }} MilestoneConfig
+ */
+
+const doc = /** @type {{ milestones?: MilestoneConfig[] } | null | undefined} */ (yaml.load(raw));
 
 if (!doc || !Array.isArray(doc.milestones) || doc.milestones.length === 0) {
   console.error('ERROR: milestones.yaml must contain a non-empty `milestones` array.');
@@ -78,12 +89,14 @@ if (extinctionMarkers.length > 1) {
 }
 
 // ── Generate JS ───────────────────────────────────────────────────────────────
+/** @param {string} s */
 function jsString(s) {
   // Emit as a template-literal-safe single-quoted JS string.
   // We use JSON.stringify for reliable escaping.
   return JSON.stringify(String(s));
 }
 
+/** @param {MilestoneConfig} m */
 function renderMilestone(m) {
   const lines = [
     '  {',
diff --git a/scripts/build-project-stats.js b/scripts/build-project-stats.js
index 655ec87..0a111ef 100644
--- a/scripts/build-project-stats.js
+++ b/scripts/build-project-stats.js
@@ -22,7 +22,7 @@ const DEST = path.join(ROOT, 'project-stats-data.js');
 
 // ── Load & parse ─────────────────────────────────────────────────────────────
 const raw = fs.readFileSync(SRC, 'utf8');
-const doc = yaml.load(raw);
+const doc = /** @type {{ pr_count?: unknown, total_tokens?: unknown } | null | undefined} */ (yaml.load(raw));
 
 if (!doc || typeof doc !== 'object') {
   console.error('ERROR: project-stats.yaml must be a valid YAML mapping.');
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..b813200
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,50 @@
+{
+  "compilerOptions": {
+    // Type-check existing .js files without emitting output.
+    // The browser still loads the original .js files directly — no build step required.
+    "checkJs": true,
+    "noEmit": true,
+
+    // Strict mode catches the most common classes of type-related bugs.
+    "strict": true,
+
+    // Target & lib: match the environments these files actually run in.
+    // DOM is included so that `window` and DOM globals are recognised in
+    // death-clock-core.js (browser execution path).
+    "target": "ES2020",
+    "lib": ["ES2020", "DOM"],
+    "module": "CommonJS",
+
+    // Explicitly include @types/node so Node.js globals (require, process,
+    // __dirname, console, fs, path …) are available in the build scripts.
+    "types": ["node", "js-yaml"],
+
+    // Allow the dual-export pattern used by death-clock-core.js
+    // (module.exports for Jest, window.* for the browser).
+    "esModuleInterop": true,
+
+    // Report errors on JS files explicitly included below.
+    "allowJs": true
+  },
+  "include": [
+    // Global type declarations for window.* extensions (dual-export pattern).
+    "global.d.ts",
+    // Pure-functions module — highest value, well-annotated with JSDoc already.
+    "death-clock-core.js",
+    // Node.js build scripts — simple file-system tools, easy to type-check.
+    "scripts/**/*.js"
+  ],
+  "exclude": [
+    "node_modules",
+    // Auto-generated files — do not edit or type-check directly.
+    "changelog-data.js",
+    "milestones-data.js",
+    "project-stats-data.js",
+    // DOM + CDN-global heavy files — deferred to a future incremental step
+    // (would require Chart.js and window.DeathClockCore type declarations).
+    "script.js",
+    "chart-date-adapter.js",
+    // Test files — checked separately by Jest.
+    "tests/**"
+  ]
+}