One moment. Three clocks.

UTC is part of the equation

UTC means Coordinated Universal Time. It is the shared reference for local clocks around the world. UTC has an offset of zero and does not change for daylight saving time.

Origin local time − origin UTC offset = UTC.
UTC + destination UTC offset = destination local time.

Combining those steps gives destination local time = origin local time + (destination offset − origin offset). That is why we use D − O. Apply the difference to the clock, then check the calendar date.

U.S. time-zone memory aid

Please Make Coffee Early: Pacific → Mountain → Central → Eastern, moving west to east. Each is usually one hour ahead of the previous zone.

Standard offsets: Pacific UTC−8, Mountain UTC−7, Central UTC−6, Eastern UTC−5. During daylight saving time: UTC−7, UTC−6, UTC−5, UTC−4. Alaska and Hawaii have their own zones; most of Arizona stays at UTC−7. Always use the date-specific offset shown above.

Global time-zone terms

Time zone

A region that shares local clock rules. Governments set boundaries and daylight saving rules; time zones do not follow longitude lines exactly.

UTC offset

The amount added to UTC to get local time. UTC+9 is nine hours ahead; UTC−5 is five hours behind. Some offsets include half-hours or quarter-hours.

IANA time-zone identifier

A geographic rule name, such as America/Chicago or Asia/Tokyo, used to calculate the correct offset for a date.

Standard time and daylight saving time

Standard time is a zone's baseline clock setting. Some places advance their clocks seasonally; others keep the same offset all year.

Time-zone abbreviation

A short label such as CST, CDT, or JST. Abbreviations can be shared by different regions, so use the numeric UTC offset for the calculation.

Calendar date and International Date Line

Each local date changes at midnight. The International Date Line is a date boundary in the Pacific; the same instant can have different local dates. Calculate the destination date along with its time.