World Time Meeting Planner

Every civil time zone is expressed as a signed offset from Coordinated Universal Time, so converting a meeting slot is a two-step operation: subtract the organizer offset to reach UTC, add each participant offset, then reduce modulo 24 and record the day shift that the reduction produced. Offsets currently in use span UTC-12 to UTC+14, a 26-hour range wider than a day, because Kiribati moved its Line Islands to UTC+14 in 1995 so the whole country would share a calendar date. Not all offsets are whole hours: India uses UTC+05:30, Iran UTC+03:30, Myanmar UTC+06:30, Newfoundland UTC-03:30, and parts of Australia UTC+09:30, while Nepal at UTC+05:45 and the Chatham Islands at UTC+12:45 use quarter-hour offsets. The overlap arithmetic is simple enough to reason about directly. If the widest gap between any two participant offsets is S hours and the acceptable window is W hours wide, the number of start hours that satisfy everyone is W minus S, or zero if S exceeds W. With business hours defined here as 09:00 to 17:00, W is 8, so any group spread beyond 8 hours has no common business slot at all; the extended window of 07:00 to 21:00 gives W equal to 14 and tolerates a spread up to 14 hours. San Francisco to Beijing at 16 hours apart therefore has no reasonable common slot in either window, and the only options force someone outside it. Three caveats apply. This planner uses fixed offsets and does not model daylight saving, so a pairing such as New York and Sydney, whose true gap oscillates between 14 and 16 hours as the two hemispheres shift in opposite directions, must be checked for the specific week. Country-level policy overrides geography, since China runs all its territory on UTC+08 and Spain sits on Central European Time despite lying at British longitude. And the day shift matters as much as the hour when the group straddles the date line.

Local solar time was adequate until vehicles moved faster than the sun. British railways began imposing London time on their networks from 1840, and the Railway Clearing House pushed the practice across the country by 1847, creating the first large-scale abandonment of local mean time. In North America the Scottish-Canadian engineer Sandford Fleming argued from 1879 for a worldwide system of hour-wide zones referenced to a single prime meridian, and the US and Canadian railroads implemented standard zones on November 18, 1883, a changeover remembered as the day of two noons. The International Meridian Conference in Washington in October 1884 then gave the scheme a global anchor by adopting Greenwich as the prime meridian and defining a universal day. Legal and technical infrastructure followed. The United States gave the zones statutory force with the Standard Time Act of 1918, which also introduced daylight saving, and reduced the resulting patchwork with the Uniform Time Act of 1966. The reference itself was upgraded from astronomical Greenwich Mean Time to atomic Coordinated Universal Time, adopted for broadcast coordination around 1960 and restructured into its present leap-second form in 1972. The practical problem for software, that offsets and daylight saving rules change by political decision and must be known historically as well as prospectively, was solved by the time zone database begun by Arthur David Olson around 1986 and long maintained by Paul Eggert. That database, transferred to IANA stewardship in 2011, is updated several times a year as governments alter their rules, and it is the authority behind the offsets that calendar and scheduling tools apply.