How Does a World Clock Work?
The Foundation: UTC
Every world clock starts with UTC — the universal time standard maintained by atomic clocks. Your device knows the current UTC time and calculates local times by applying UTC offsets.
UTC Offsets
Each time zone has a UTC offset:
- New York (EST): UTC-5
- London (GMT): UTC+0
- Tokyo (JST): UTC+9
- India (IST): UTC+5:30
The IANA Time Zone Database
The IANA tz database contains:
- Historical time zone data
- Current UTC offsets
- DST rules (when clocks change, by how much)
- Political changes to time zones
Every OS, browser, and programming language uses this database.
Real-Time Updates
World clocks update every second by recalculating local times from the current UTC time — automatically handling DST transitions.
Quick Answer
A world clock takes the current UTC time, applies UTC offsets from the IANA database (including DST rules), and displays local times for each location.
