From 4a9955be7dd6405aa2500d0012f2993db0c11872 Mon Sep 17 00:00:00 2001
From: Jethro Nederhof <jethro.nederhof@snowplowanalytics.com>
Date: Fri, 30 Aug 2024 14:18:00 +1000
Subject: [PATCH] Add browser-tracker-core default for os_timezone

---
 .../ostz_default_2024-08-30-04-14.json               | 10 ++++++++++
 libraries/browser-tracker-core/src/helpers/index.ts  | 12 ++++++++++++
 libraries/browser-tracker-core/src/tracker/index.ts  |  3 +++
 3 files changed, 25 insertions(+)
 create mode 100644 common/changes/@snowplow/browser-tracker-core/ostz_default_2024-08-30-04-14.json

diff --git a/common/changes/@snowplow/browser-tracker-core/ostz_default_2024-08-30-04-14.json b/common/changes/@snowplow/browser-tracker-core/ostz_default_2024-08-30-04-14.json
new file mode 100644
index 000000000..9de6e47f1
--- /dev/null
+++ b/common/changes/@snowplow/browser-tracker-core/ostz_default_2024-08-30-04-14.json
@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@snowplow/browser-tracker-core",
+      "comment": "Add browser-tracker-core default for os_timezone",
+      "type": "none"
+    }
+  ],
+  "packageName": "@snowplow/browser-tracker-core"
+}
\ No newline at end of file
diff --git a/libraries/browser-tracker-core/src/helpers/index.ts b/libraries/browser-tracker-core/src/helpers/index.ts
index 817780a86..6c93b2b07 100755
--- a/libraries/browser-tracker-core/src/helpers/index.ts
+++ b/libraries/browser-tracker-core/src/helpers/index.ts
@@ -81,6 +81,18 @@ export function isFunction(func: unknown) {
   return false;
 }
 
+/**
+ * Lightweight configured runtime timezone detection
+ * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat/resolvedOptions#getting_the_users_time_zone_and_locale_preferences
+ * @returns IANA timezone name of current runtime preference
+ */
+export function getTimeZone(): string | void {
+  if (typeof Intl === 'object' && typeof Intl.DateTimeFormat === 'function') {
+    const systemPreferences = new Intl.DateTimeFormat().resolvedOptions();
+    return systemPreferences.timeZone;
+  }
+}
+
 /**
  * Cleans up the page title
  */
diff --git a/libraries/browser-tracker-core/src/tracker/index.ts b/libraries/browser-tracker-core/src/tracker/index.ts
index 730972aa9..e4b93a322 100755
--- a/libraries/browser-tracker-core/src/tracker/index.ts
+++ b/libraries/browser-tracker-core/src/tracker/index.ts
@@ -27,6 +27,7 @@ import {
   attemptGetSessionStorage,
   attemptWriteSessionStorage,
   createCrossDomainParameterValue,
+  getTimeZone,
 } from '../helpers';
 import { BrowserPlugin } from '../plugins';
 import { newOutQueue } from './out_queue';
@@ -306,6 +307,7 @@ export function Tracker(
     }
 
     const { browserLanguage, resolution, colorDepth, cookiesEnabled } = getBrowserProperties();
+    const timeZone = getTimeZone();
 
     // Set up unchanging name-value pairs
     core.setTrackerVersion(version);
@@ -317,6 +319,7 @@ export function Tracker(
     core.addPayloadPair('lang', browserLanguage);
     core.addPayloadPair('res', resolution);
     core.addPayloadPair('cd', colorDepth);
+    if (timeZone) core.addPayloadPair('tz', timeZone);
 
     /*
      * Initialize tracker