Time Zones, UTC and Daylight Saving: A Practical Guide

Time zones look simple until you schedule a meeting across three countries or store a date in a database. A few ideas clear up most of the confusion.

What UTC is

UTC, or Coordinated Universal Time, is the world's common reference. It doesn't change with the seasons. Every other zone is described as an offset from it, such as UTC+1 for Lagos or UTC-5 for New York in winter.

Offsets aren't always whole hours

• India is UTC+5:30.
• Nepal is UTC+5:45.
• Parts of Australia use a half-hour offset.
• Some places don't change at all, while others shift on different dates.

Daylight saving time

Many countries move their clocks forward in spring and back in autumn. The dates are different in different regions, so the gap between two cities can change twice a year, and for a few weeks it can be different from what you expect. For example, New York is UTC-5 in winter and UTC-4 in summer. Many countries, including Nigeria, India and Japan, don't use daylight saving at all.

Use the place name, not an abbreviation

Abbreviations such as CST or IST are ambiguous, because each one can mean several different zones. A proper name like America/Chicago or Asia/Kolkata identifies the exact rules, including the history of daylight saving changes. These are called IANA time zone names.

Rules that save you trouble

1. Store and send times in UTC, and convert to local time only when you display them.
2. When you schedule a meeting, include the zone or a link that converts it for each reader.
3. Don't add or subtract hours by hand across a daylight saving change. Use a tool that knows the rules.
4. Be careful with meetings near the clock change dates.

Tools

The World Clock shows the time in several places and converts a date and time from one zone to the rest, taking daylight saving into account. Developers can use the Unix Timestamp Converter to turn timestamps into dates in any zone, and the Date Calculator to count days between two dates.