Time Zone

Current time in major cities

These clocks show the current local time and take the city’s seasonal rules into account. They are more reliable than a remembered time difference that may not apply throughout the year.

What is a time zone and why do we have them?

Earth rotates through 360 degrees in about 24 hours. Dividing 360 by 24 gives 24 theoretical slices, each 15 degrees wide. Each slice could use the solar time of its central meridian: travel east and the clock advances; travel west and it moves back.

The model and real life

The 24 slices explain the principle, but civil time follows national borders, economic needs and political decisions. This creates half-hour and quarter-hour offsets and allows very large countries to use one official time.

The reference meridian passes through Greenwich in London. GMT means Greenwich Mean Time and remains the name of a time zone; UTC is the international atomic time scale coordinated worldwide. GMT and UTC both have a zero offset in everyday use, but they are not technically the same definition.

Civil time is not set by longitude alone. Spain, France, Italy, Germany and Poland share Central European Time despite covering very different longitudes. Santiago de Compostela and Warsaw have the same official time although they are almost 2,500 kilometres apart, so sunset by the clock is much later in western Spain.

To explore how longitude and Earth’s axial tilt affect sunrise and sunset, see the guide to the shortest and longest days of the year.

Time-zone curiosities

• China spans roughly five theoretical zones but uses one official time, UTC+8 or Beijing time.
• Australia uses several standard zones, including UTC+8, UTC+9:30 and UTC+10. Queensland, the Northern Territory and Western Australia do not observe daylight saving time.
• India and Nepal show that offsets need not be whole hours: India uses UTC+5:30 and Nepal UTC+5:45.
• Turkey uses UTC+3 throughout the year and does not change its clocks seasonally.

An unusual sequence appears when travelling east: China is UTC+8, Vladivostok UTC+10 and Japan UTC+9. The clock therefore moves forward by two hours and then back by one, because political time-zone boundaries interrupt the geometric sequence of the meridians.

Italy’s time zone

Italy adopted Central European Time on 1 November 1893, based on the 15° east meridian that crosses the Termoli–Etna axis. The country now uses CET (UTC+1) during standard time.

During daylight saving time Italy uses CEST (UTC+2). European Union countries move their clocks forward on the last Sunday in March and return to standard time on the last Sunday in October. The proposal to end the seasonal changes has not become law, so the present system remains in place.

The abbreviations UTC, PST, MST, CST, EST and CET

Abbreviations describe a standard or seasonal time, but they do not always identify one location unambiguously. Travel bookings and computer systems are safer with geographic identifiers such as Europe/Rome or America/New_York, maintained in the IANA database.

During daylight saving time the North American abbreviations become PDT, MDT, CDT and EDT, each one hour ahead; Central Europe uses CEST, UTC+2. CST can also mean China Standard Time in another context, which is another reason to include a city and date.

Rapidly crossing several zones can disrupt sleep, attention and appetite. This is jet lag; gradually adjusting sleep and light exposure and beginning to think in destination time during the journey can help.

Calendars and working across time zones

When I arrange an international call, I first check the local time for everyone and look for a reasonable overlap. In the invitation I write the full time, the reference zone or city and allow the calendar to convert it for each participant.

• check the city and date, not only a remembered time difference;
• find out whether and when each country changes its clocks;
• use the 24-hour format to reduce ambiguity;
• state the zone in the invitation, for example “18:00 Europe/Rome”;
• avoid meetings that fall during the night for either side.

Setting time zones in Google Calendar

On a computer, open Settings → General → Time zone to choose the primary zone and display a secondary one. The World clock section can show more cities, while an individual event can have its own zone selected beside the time.

Time zones for travel destinations

Find the current time, time zone and daylight saving rules for your destination.

Flags: flag-icons · Panayiotis Lipiridis, MIT.

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