How Do Time Zones Work? A Plain-Language Explanation
Time zones divide the world into regions that share the same local time. Understanding offsets, UTC, and daylight saving time helps you schedule across locations without making costly mistakes.
Why Time Zones Exist
Before standardized time zones, every city kept its own local time based on when the sun reached its highest point in the sky. Noon in one city might be 12:07 or 11:54 in a nearby city just a few miles east or west. This worked fine when travel was slow, but the expansion of railroads in the 19th century made inconsistent local times a scheduling nightmare.
In 1884, the International Meridian Conference in Washington, D.C. established Greenwich, England as the prime meridian and divided the world into 24 standard time zones, each spanning 15 degrees of longitude. Since Earth rotates 360 degrees in 24 hours, each hour of rotation covers 15 degrees.
Today, most of the world follows this system, though political and practical considerations mean actual time zone boundaries often follow national or regional borders rather than pure longitude lines. China, for example, spans roughly four natural time zones but uses a single national time for the entire country.
Understanding UTC and Offsets
Coordinated Universal Time, or UTC, is the global reference point for all time zones. It is the modern successor to Greenwich Mean Time and is based on atomic clocks rather than the position of the sun. Every time zone in the world is expressed as an offset from UTC, either ahead (positive offset) or behind (negative offset).
New York during Eastern Standard Time is UTC minus 5, meaning its clocks show a time 5 hours behind UTC. Tokyo is UTC plus 9, meaning its clocks show a time 9 hours ahead of UTC. The total difference between New York Standard Time and Tokyo time is 14 hours.
Not all offsets are whole hours. India Standard Time is UTC plus 5:30, Nepal Standard Time is UTC plus 5:45, and several other regions use 30-minute or 45-minute offsets. When scheduling across these regions, remember to account for the fractional hour difference.
Daylight Saving Time Complications
Daylight saving time, or DST, is a practice where clocks are moved forward one hour in spring and back one hour in autumn. The goal is to shift an hour of daylight from the early morning, when most people are asleep, to the evening, when people are more active. Not all countries observe DST, and those that do may change clocks on different dates.
The United States moves clocks forward on the second Sunday in March and back on the first Sunday in November. The European Union moves clocks on the last Sunday in March and the last Sunday in October. During the weeks when one region has changed clocks but another has not, the offset between them temporarily differs from its standard value.
This creates genuine confusion for international scheduling. A regular weekly call between New York and London that normally spans 5 hours of difference can temporarily span only 4 hours for a few weeks each spring and autumn. Always check current UTC offsets when scheduling across regions that may be in different phases of DST.
Converting Between Time Zones
The most reliable way to convert between time zones is to first convert the original time to UTC, then convert from UTC to the target time zone. If it is 3:00 PM in Los Angeles (UTC minus 8 during standard time), converting to UTC gives 11:00 PM UTC. Converting 11:00 PM UTC to Singapore time (UTC plus 8) gives 7:00 AM the following day.
The date can change during conversion, which catches many people off-guard. A meeting scheduled for 9:00 PM Monday in San Francisco (UTC minus 8) occurs at 1:00 PM Tuesday in London (UTC plus 0 during GMT) and at 2:00 PM Tuesday in Paris (UTC plus 1). Participants in later time zones may be joining on a different calendar day.
Time zone conversion tools and world clocks handle these calculations instantly. Many calendar applications, including Google Calendar and Outlook, automatically display meeting times in each participant's local time zone when you add guests from different regions.
Practical Tips for Scheduling Across Time Zones
When scheduling international meetings, choose a time that falls within normal working hours for as many participants as possible. A meeting at 9:00 AM Pacific time is 5:00 PM in London and midnight in Tokyo. Finding a time that works for Tokyo without requiring someone to join at 2:00 AM often requires creative scheduling or rotating meeting times across participants.
Always specify the time zone explicitly when communicating meeting times, deadlines, or schedules across regions. Writing '3:00 PM ET' or '3:00 PM UTC minus 5' eliminates ambiguity. Avoid writing just '3:00 PM' in any communication that crosses regional boundaries.
For product launches, financial transactions, or event coordination that spans multiple countries, consider using UTC as a shared reference time in all communications. Everyone can convert from UTC to their own local time, and there is no risk of confusion about whose time zone the stated time refers to.