Coordinated Universal Time (UTC) is the global time standard. All other time zones are defined as offsets from UTC, ranging from UTC−12:00 to UTC+14:00 — a spread of 26 hours, which is why some date/time edge cases are complex. A UTC offset of +05:30 means clocks in that zone read 5 hours and 30 minutes ahead of UTC. India's offset is +05:30, Nepal's is +05:45, creating a 15-minute difference between neighboring countries.
If time zones were purely based on longitude (15° of longitude per hour), there would be 24 neat, even-width vertical slices. In practice, political and economic convenience distorts the boundaries heavily. China spans 62 degrees of longitude — theoretically 4+ time zones — but operates entirely on UTC+8 for national unity. Spain, which is geographically in the same longitudinal band as the UK (UTC+0), uses UTC+1 for historical alignment with continental Europe.
Daylight Saving Time: what changes and what doesn't
Daylight Saving Time (DST) shifts clocks forward by one hour in spring and back by one hour in fall, in regions that observe it. The effect is that sunset is an hour later in summer evenings. UTC does not observe DST — it is always UTC. Regions that observe DST have two UTC offsets depending on the time of year: for example, the US Eastern Time zone is UTC−5 in winter (Eastern Standard Time, EST) and UTC−4 in summer (Eastern Daylight Time, EDT).
Not all countries observe DST. Japan, China, India, and most of Africa do not. The US, most of Europe, Australia, and parts of South America do. The transition dates are also not synchronized: the EU switches on the last Sunday of March, the US on the second Sunday of March. This means there's a two-to-three week period each spring when the offset between US and European time zones is different than usual, which causes scheduling errors for international meetings booked months in advance.
Why time zone handling is hard in software
Storing timestamps in UTC and converting to local time only for display is the universal correct approach in software development. Storing timestamps in local time is dangerous because DST transitions create ambiguous times (when clocks fall back, 1:30 AM happens twice) and gaps (when clocks spring forward, 2:30 AM never exists). UTC has neither problem.
The IANA Time Zone Database (also called the tz database or Olson database) is the authoritative reference for the world's time zone rules. It's used by every major operating system and programming language. It's updated several times a year as countries change their DST rules or offsets — which happens more often than most people realize. Countries have changed their time zone offset or DST rules over 600 times since 1970.