Every place keeps time as an offset from UTC, and that offset can change twice a year if the region observes daylight saving. Pick a date and two places, and the conversion accounts for whatever rules applied on that specific date — not just today's offset, which is why scheduling something six months out can trip people up.
A 9am meeting in London in January (GMT, UTC+0) lands at 1am in Los Angeles (PST, UTC−8) — an 8-hour gap. Schedule the same 9am London meeting for July instead, and it's 1am Pacific Daylight Time, because both zones have shifted for daylight saving, but not always by the same amount or on the same date.
Daylight saving mainly benefits places with a big seasonal swing in daylight hours — mid-to-high latitudes. Countries near the equator see little seasonal daylight change, so many skip it entirely, while others have abandoned it for practical or political reasons.
Time zones are nominally 15° of longitude (1 hour) wide, but borders follow countries, not meridians. A handful of places — India, Iran, parts of Australia — set their offset a half or even three-quarter hour off the nearest whole hour, usually to sit in the middle of a wide country's longitude span.
They come from your browser's built-in time-zone database (the IANA tz database), which is updated alongside your browser or operating system. That means the rules are as current as your device — a long-unpatched browser could carry outdated transition dates.
Yes — governments have moved daylight-saving start/end dates, or abolished DST altogether, sometimes with only a few months' notice. For anything truly time-critical, especially in the near future, it's worth double-checking against an official government or airline source.