How Time Zones Work

A time zone is a region of the Earth that keeps the same standard time. In principle the globe divides into 24 zones, one for each hour, but in practice zones follow country and state borders — so the real map is far more irregular than a neat set of stripes.

From longitude to zones

The Earth rotates 360° in 24 hours, or 15° of longitude per hour. That is the idea behind time zones: each 15°-wide slice is roughly one hour apart from its neighbours, all measured from the prime meridian at Greenwich.

Reality bends the theory. Countries keep a single zone for convenience even when they span several — China uses one time zone across a span that "should" be five. Others use half-hour or 45-minute offsets, like India (UTC+5:30) and Nepal (UTC+5:45).

The IANA time zone database

Software does not track raw offsets; it uses the IANA time zone database, which records named zones like "America/New_York" or "Asia/Tokyo" along with their full history of offset and daylight-saving changes.

This is why a zone is written as Region/City rather than as a fixed "UTC−5". The named zone knows when that place switches for daylight saving and even when its rules changed in the past.

The International Date Line

On the far side of the globe from Greenwich runs the International Date Line, where the calendar day changes. Cross it going west and you skip forward a day; cross it going east and you repeat one. It zigzags to keep island nations on a single date.