Understanding Time Zones

The world is divided into 24 primary time zones, each roughly 15 degrees of longitude wide. In practice, time zone boundaries follow political borders rather than neat meridian lines, which is why the map looks more like a jigsaw puzzle than a grid.

How Time Zones Work

All time zones are defined as offsets from Coordinated Universal Time (UTC, formerly GMT). New York is UTC-5 in winter and UTC-4 during daylight saving time. Tokyo is UTC+9 year-round. The International Date Line, roughly along the 180th meridian in the Pacific, is where the calendar date changes.

Daylight Saving Time

Not all countries or regions observe DST. In the US, most states spring forward one hour in March and fall back in November. Arizona (except the Navajo Nation) does not observe DST. Neither does Hawaii. The EU observes DST from March to October. This means the offset between two cities can change depending on the time of year.

Half-Hour and Quarter-Hour Offsets

Some time zones use 30- or 45-minute offsets. India is UTC+5:30. Nepal is UTC+5:45. The Chatham Islands in New Zealand use UTC+12:45. These unusual offsets exist because countries chose the offset that best matched their geographic position.

Tips for Scheduling Across Time Zones

• Always specify the time zone when scheduling across regions (e.g., "3 PM ET" not just "3 PM")
• Use UTC as a neutral reference when coordinating with people in many different zones
• Be aware of the date difference -- it can already be tomorrow in Sydney while it is still today in Los Angeles