A date is not a fixed duration
A calendar label answers which local date an instant belongs to. A duration answers how much clock time separates two instants. Changing an offset can move those boundaries without changing the written date.
In the independently checked London example, 29 March 2026 runs from 00:00 UTC to 23:00 UTC: 23 hours. The 25 October date runs from 23:00 UTC on 24 October to 00:00 UTC on 26 October: 25 hours. These are worked time-zone calculations, not two different astronomical definitions of Earth’s rotation.
When midnight is missing or repeated
A civil-day search must not assume that 00:00 occurs exactly once. The São Paulo fixture for 4 November 2018 begins at the first valid time after its midnight clock jump; the Havana fixture for 1 November 2020 includes the repeated midnight hour.
Some dates disappear altogether after date-line changes. Other historical dates can occur in separated UTC intervals. Atlas reports those unavailable for a single civil-day window rather than filling in the missing period. Use an explicit time window if that better represents the historical question.
Choose a day for the question
Use a civil day when you want everything recorded under a particular local date. Use fixed 24 hours when comparing equal-length windows from a known instant. Use sunrise to sunrise when the day is defined by two solar events.
In the paran explorer, changing this choice recalculates the actual boundaries and matching pairs. The downloaded record keeps the mode, UTC boundaries, duration and geometric conventions. Local clock rules come from the runtime’s IANA-backed date formatter; minute samples locate date transitions and the boundary is refined to one millisecond. This numerical resolution does not prove the historical clock record.
IANA: theory and scope of the time-zone database ↗Zyntara: day conventions and actual moving bodies ↗
Word origin · etymology
English civil + day; civil comes through French from Latin civilis.
Civilis derives from civis, citizen. Here civil identifies society’s clock/calendar convention; the compound’s earliest technical use is not established by this entry.
How we know · epistemology
Calendar convention and historical time-zone records.
The date’s boundaries depend on the chosen time-zone rules. IANA cautions that its records do not establish every local historical clock practice, especially before 1970.
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Connected concepts
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Mentioned in 2 other entries
These links connect subjects and terminology. They do not establish a shared historical origin.
Sources & provenance
Use the citations beside each section to distinguish a calculation convention, a historical teaching and an editorial example.