Time Zone Transition Local Fold Span

Description

Derive a single transition’s local fold for a negative jump as a half-open interval; other jump signs return null.

An offset maps a supplied UTC coordinate to local civil coordinates by local = UTC + offset. Half-open spans include their start and exclude their end. A forward offset jump can leave local coordinates uncovered; a backward jump can give several UTC interpretations of one local value. These tools use explicit synthetic civil-second observations rather than a named-zone database.

Derive a single transition’s local fold for a negative jump as a half-open interval; other jump signs return null.

When to use this tool

Use this tool to derive a single transition’s local fold for a negative jump as a half-open interval; other jump signs return null.

How to use the tool and how it works

Enter at, before, after using the definitions below.
Run the operation. Resolve input-format errors before interpreting an output.
Read result.span, using the stated boundary, ordering, sign and absence conventions.

When after<before return the half-open local interval [at+after,at+before). Otherwise return span=null.^[1]

Input field	Meaning and units	Accepted input
at	UTC transition instant in seconds within ±10¹².	Finite numeric value satisfying the stated comparisons.
before	Offset immediately before the transition, in seconds within ±86,400.	Finite numeric value satisfying the stated comparisons.
after	Offset effective at and after the transition, in seconds within ±86,400.	Finite numeric value satisfying the stated comparisons.

Output field	Interpretation
result	Object containing the fields below.
result.span	Half-open local gap/fold {start,end} in seconds, or null when that offset-change direction does not apply.

json

{
  "input": {
    "at": 10,
    "before": 2,
    "after": 0
  },
  "output": {
    "result": {
      "span": {
        "start": 10,
        "end": 12
      }
    }
  }
}

Limitations and assumptions

The repeated local interval comes from one supplied backward-offset transition; other span observations can create additional candidates.
Span/transition coordinates are finite seconds within ±10¹² and offsets within ±86,400 seconds. Spans require start<end. No IANA database, calendar parsing, daylight-saving inference or leap seconds are included.
Every declared outer field is required and unknown outer fields are rejected. Strings and JSON text have size limits, lists are bounded to 128 entries, and JSON nesting/node counts are bounded. Tighter operation-specific bounds stated above take precedence.
Ordinary numeric outputs must be finite and within safe-integer magnitude. Exact large lengths/counts use decimal strings. Original record indexes and code-unit positions are zero-based; span ends are exclusive unless stated otherwise.

Alternative or Complementary methods

Keep all timestamps on one origin and in seconds, and retain gaps or competing observations. Use a maintained time-zone library when real dates, named zones or historical rules are required.

References

Tool behavior and example: Time Zone Transition Local Fold Span — tool.io

Similar or alternative tools

Time Zone Transition Local Gap Span
Derive a single transition’s local gap for a positive jump as a half-open interval; other jump signs return null.
Time Zone UTC Span Local Split
Intersect UTC windows with every supplied offset span and map each piece independently into local coordinates; gaps/overlaps are not concealed.