{
  "schema_version": "1.3.0",
  "slug": "when-does-a-wall-clock-schedule-fire-twice-or-zero-times-on-a-dst-transition",
  "title": "When does a wall-clock schedule fire twice or zero times on a DST transition?",
  "date_of_record": "2026 November 1",
  "scope": "Applies to recurring jobs scheduled on local wall-clock time (daily at 02:30) in a timezone that observes daylight saving. Excludes UTC-scheduled jobs and instant-based schedulers.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-does-a-wall-clock-schedule-fire-twice-or-zero-times-on-a-dst-transition.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-does-a-wall-clock-schedule-fire-twice-or-zero-times-on-a-dst-transition.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-does-a-wall-clock-schedule-fire-twice-or-zero-times-on-a-dst-transition.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 927,
    "python_slice_tokens": 155,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "**Predicate.** A wall-clock schedule is a total function from date to instant if and only if every date maps to exactly one instant. On a spring-forward transition, local times in the skipped hour do not exist and map to zero instants. On a fall-back transition, local times in the repeated hour map to two instants. A schedule is sound only if it is expressed...",
    "what_was_observed": "**Signature.** Duplicate output for a daily job on one specific date per year, or a missing day. The date matches the timezone's DST transition. Re-running the job manually for the affected date does not reproduce the duplicate, which makes it look like a one-off. **Trace.** A job scheduled for 02:30 local. On fall-back, 02:30 occurs twice, an hour apart. A scheduler that triggers on wall-clock equality fires at both...",
    "condition_of_validity": "A recurring schedule is unambiguous only while at least one holds: 1. **UTC.** The schedule is expressed in UTC and the local-time intent is accepted to drift by an hour twice a year. 2. **Explicit policy.** The scheduler stores absolute instants and has a declared rule for nonexistent and repeated local times (skip, shift, or fire-once). 3. **Idempotence.**...",
    "decision_boundary": "- UTC schedule: fires exactly once per period; local time shifts. - Wall-clock with stored absolute instants and a declared policy: fires exactly once by construction. - Wall-clock with recomputation each tick: double or zero fire is guaranteed on transitions. - Wall-clock plus idempotent job: safe for that job, unsafe for any new job added to the same sched...",
    "operational_effect": "The defect is concentrated in two instants per year and therefore invisible to any test that does not cross a transition. Remediation is to store next-fire instants rather than recompute wall-clock times, and to make the logical date explicit in the job payload. The trap is to move the schedule to a time outside the transition window. That works until a time..."
  },
  "matched_failure_symptoms": [
    "DOMAIN:EPISTEMIC_PROVENANCE_AND_FEDERATION_ROUTING"
  ],
  "composition_edges": {
    "primary_domain": "epistemic_provenance_and_federation_routing",
    "upstream_precondition_residuals": [
      "https://corpusfetch.com/c/according-to-what-precedence-hierarchy-must-an-instance-resolve-contradictory-directives-across-system-developer-and-user-tiers.md",
      "https://corpusfetch.com/c/how-should-an-instance-decide-that-a-stored-conclusion-has-gone-stale-without-a-new-observation.md"
    ],
    "downstream_consequence_residuals": [
      "https://corpusfetch.com/c/by-what-checkpoint-and-compaction-boundaries-must-an-instance-externalize-working-state-to-prevent-context-window-drift-and-truncation-amnesia.md",
      "https://corpusfetch.com/c/across-what-additive-schema-and-unknown-field-preservation-boundaries-must-an-instance-evolve-wire-contracts-to-prevent-rolling-deploy-deserialization-crashes.md"
    ],
    "co_occurring_sibling_residuals": [
      "https://corpusfetch.com/c/how-can-an-instance-capture-implicit-user-preferences-without-storing-private-data.md",
      "https://corpusfetch.com/c/how-can-an-instance-compress-long-reasoning-traces-into-binding-conclusions.md"
    ]
  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "13f68f2ded0d133c37c25867da7686559dcd1e0a9f9c74c5dd32bc7469bc98ce",
    "verified_classes": [
      "ScheduleAmbiguityError",
      "ScheduleGate"
    ],
    "verified_functions": [
      "dispatch_schedule",
      "verify"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 2,
    "raised_exceptions": [
      "ScheduleAmbiguityError"
    ],
    "sample_fail_closed_invariants": [
      "wall-clock schedule with no DST policy fires twice or zero times at transitions"
    ],
    "stdlib_dependencies": [
      "dataclasses"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}