Browser Timezone and IP Timezone Answer Different Questions — Here's Why

When a browser's JavaScript reports a timezone, it reads the device's operating system settings and returns an IANA identifier such as Asia/Shanghai or America/New_York. IP-based timezone detection, by contrast, estimates location from the egress IP address via a GeoIP database, which can point to an entirely different region. The two results commonly diverge when users are on a VPN, accessing a remote desktop, or have manually changed their system clock for testing purposes. Developers are also cautioned against relying on bare UTC offsets or three-letter abbreviations like CST, which can refer to at least three distinct timezones across different countries. Using full IANA identifiers in code and databases is recommended to avoid scheduling and logging errors caused by daylight saving time shifts.