Type: JavaScript API (Intl.DateTimeFormat().resolvedOptions().timeZone)
Data Source: Operating System / System Clock Settings
When Available: Always accessible via JavaScript (modern browsers)
Return Type: String (IANA timezone identifier)
Background & Purpose
The timezone detection API returns the IANA timezone identifier (also called Olson timezone) configured in the user's operating system. This provides precise geographic and temporal information about the user's location.
Historical Context:
Originally, only timezone offset (in minutes) was available via Date.getTimezoneOffset()
The Intl.DateTimeFormat API introduced proper timezone name detection
Based on the IANA Time Zone Database (formerly Olson database), maintained since 1986
Essential for displaying accurate local times and scheduling across regions
Standardized across modern browsers as part of ECMAScript Internationalization API
Why It Matters:
Enables accurate time display without server-side detection
Critical for scheduling applications, calendars, and meeting tools
Handles daylight saving time (DST) transitions automatically
More precise than IP-based geolocation for timezone purposes
IANA Timezone Format & Examples
Format: Timezones follow the pattern Area/Location or Area/Region/Location
Common Examples:
IANA Timezone
Region
UTC Offset (Standard)
DST
America/New_York
US Eastern Time
UTC-5
Yes (EDT: UTC-4)
America/Los_Angeles
US Pacific Time
UTC-8
Yes (PDT: UTC-7)
America/Chicago
US Central Time
UTC-6
Yes (CDT: UTC-5)
Europe/London
United Kingdom
UTC+0
Yes (BST: UTC+1)
Europe/Paris
Central European Time
UTC+1
Yes (CEST: UTC+2)
Asia/Tokyo
Japan
UTC+9
No
Asia/Shanghai
China
UTC+8
No
Australia/Sydney
Eastern Australia
UTC+10
Yes (AEDT: UTC+11)
America/Sao_Paulo
Brazil
UTC-3
Varies by year
Asia/Dubai
UAE
UTC+4
No
Note: There are over 400 timezone identifiers in the IANA database, covering historical and regional variations.
Common Uses
Legitimate Uses:
Event Scheduling: Display meeting times in user's local timezone
Content Timing: Show "posted 2 hours ago" relative to user's time
E-commerce: Display delivery estimates in local time
Analytics: Understand when users are most active
Regional Features: Enable features available in specific regions
Communication Apps: Show accurate timestamps in chats and emails
Travel Applications: Display departure/arrival times correctly
Financial Services: Show market open/close times and trading hours
Technical Example:
A video conferencing app uses timezone to display "Tomorrow at 2:00 PM your time" instead of requiring users to manually convert times.
Platform Differences
How Timezone is Determined:
Platform
Timezone Source
Update Behavior
Windows
System Settings > Time & Language > Date & Time
Auto-updates if "Set time zone automatically" enabled
macOS
System Preferences > Date & Time > Time Zone
Auto-updates based on location if enabled
Linux
/etc/timezone or system configuration
Manual or via NTP services
iOS
Settings > General > Date & Time
Auto-updates using location services
Android
Settings > System > Date & Time
Auto-updates using network or GPS
Browser Behavior:
All modern browsers read timezone from the operating system
Changes to system timezone may require browser restart to reflect
Tor Browser masks timezone (returns UTC or generic timezone)
Some privacy extensions may spoof timezone information
Privacy Implications
Tracking Risk:HIGH
Privacy Concerns:
Precise Geolocation: Timezone directly reveals geographic region, often narrowing location to specific cities
Combined Fingerprinting: Extremely effective when combined with language, screen resolution, and other attributes
Cross-Site Tracking: Persistent identifier that rarely changes, enabling tracking across websites
Travel Detection: Changes in timezone can reveal when users are traveling
Behavioral Profiling: Reveals active hours, work patterns, and lifestyle
VPN Detection: Mismatched timezone and IP address location can reveal VPN usage
Database Matching: Only ~400 possible values makes timezone highly effective for fingerprinting
Uniqueness Analysis:
US Eastern Time (America/New_York): Shared by ~50 million users
Combined with language and offset: Narrows to small percentage of internet users
Uncommon timezones (Pacific islands, specific regions): Very high uniqueness
Why High Risk:
Directly reveals geographic location without requiring permissions
Cannot be avoided for legitimate time-based functionality