How Arcus works out the offset
The place gives the zone, and the date decides what that zone's clock read at that moment, daylight saving included.
Recent dates are settled and every program agrees on them. Older ones are the work. The time zone database that phones and computers share is thin before 1970, and some of its older detail is folded away rather than kept. So Arcus carries its own tables and reads them first:
- 142 zones whose own history before 1970 the shared database no longer answers for, and which would otherwise be given a neighbouring country's clock.
- American zone boundaries back to 1919. The lines moved, so a county on Central time in 1946 can be on Eastern time today. 130 dated areas.
- American daylight saving from 1918 to 1966, when each state, county and city decided for itself, and war time ran unbroken from February 1942 to September 1945. 4,790 rules across 2,206 places.
- The rest of the world, 109 zones across 326 regions, from published historical research. It includes the Vichy free zone of 1940 to 1942, and the left bank of the Rhine under French occupation after the first war.
These load with the ephemeris, before any chart can be cast, so an import of several hundred charts is held until they are there. A chart saves the moment it was cast on, and getting that wrong is permanent.
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