Time, Dates, and Time Zones
Time, dates, and time zones are the rules software uses to describe when something happened and when it should happen. They prevent local clocks, changing daylight-saving rules, and ambiguous date formats from changing the meaning of a record or schedule.
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Intro
Time, Dates, and Time Zones
Time data answers two different questions. An instant answers when something happened on a shared timeline. A local date and time answers what a clock and calendar showed in a place.
Keep those questions separate. An audit event, log entry, and completed payment need an instant. A meeting at nine in Rome or a recurring business cutoff needs local calendar meaning as well.
The basic model
UTC is the shared reference used by Internet timestamps. A UTC timestamp such as 2026-07-22T14:00:00Z identifies one instant. The Z means a zero offset from UTC.
A UTC offset is a number such as +02:00 or -04:00. It states the difference between a local clock and UTC at that instant. It does not state the rules for that place tomorrow.
A time zone is a named rule set for a place or region. It maps an instant to a local date, local time, and offset. The IANA Time Zone Database supplies names such as Europe/Rome and records offset and daylight-saving changes for representative locations.
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Sources
- https://www.iana.org/time-zones
Supports
- The IANA Time Zone Database contains machine-readable code and data for local-time history
- IANA updates the database for political changes to boundaries, offsets, and daylight-saving rules
- Software and operating systems distribute time-zone-data updates to users
- https://data.iana.org/time-zones/tz-link.html
Supports
- IANA database entries represent locations with shared civil-time clocks since 1970
- IANA names are typically organized by region and representative city
- The database records offset history and daylight-saving use
- Government rule changes and downstream update delays affect future local-time behavior
- https://www.iso.org/iso-8601-date-and-time-format.html
Supports
- ISO 8601 presents numeric dates in year-month-day order
- ISO 8601 covers UTC, local time with UTC offset, date-time values, intervals, and recurring intervals
- Ambiguous numeric dates can have different interpretations
- https://www.rfc-editor.org/rfc/rfc3339
Supports
- RFC 3339 defines an Internet profile of ISO 8601 date-time syntax
- UTC improves interoperability when local daylight-saving rules can change
- Numeric offsets are local time minus UTC
- Alphabetic offset labels have caused interoperability problems
- A timestamp uses Z or a numeric offset
- https://www.rfc-editor.org/rfc/rfc9557
Supports
- A time zone maps timestamps to UTC offsets and local time
- Local times near offset changes can map to zero or multiple timestamps
- An offset does not define local-time rules for related timestamps
- Named IANA zones can change as the IANA database records rule changes
- Offset and time-zone information can become inconsistent for future local times