Countdown / Days-Until Timer mechanism

Model scope

The core subtraction is correct for two timezone-aware instants, but the catalog entry needs an explicit timezone/DST policy, output breakdown definition, precision/rounding policy, and past-target behavior before it can pass. The honest label is currently 'none'; it should disclose dependence on the device clock and timezone interpretation. The optional EventKit hook is not needed for the formula, but calendar-event timezone semantics must match the timer semantics if implemented.

• The subtraction formula delta = target_datetime − now is mathematically standard, but it is incomplete unless both datetime values are interpreted as timezone-aware instants on the same timeline.
• The specification does not define how delta is converted into the advertised remaining-time breakdown of days and hours.
• The specification does not define whether a target in the past produces a negative countdown, clamps to zero, or displays an expired state.
• The specification does not define daylight-saving-time behavior, which can make a calendar-day difference differ from a fixed 24-hour duration.

Units and conventions

• Test calculations use elapsed SI seconds, with 1 day = 86400 seconds, 1 hour = 3600 seconds, and 1 minute = 60 seconds.
• Datetime inputs are interpreted as ISO 8601 timestamps with explicit timezone offsets and converted to UTC before subtraction.
• Breakdowns below use signed total-duration decomposition: delta_seconds = days*86400 + hours*3600 + minutes*60 + seconds.
• The device clock is assumed correct and monotonic during each calculation; leap seconds are ignored.
• A production specification must explicitly choose whether 'days' means fixed 24-hour periods or calendar-day boundaries in a named timezone.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "target_datetime": "2025-03-10T12:45:00+00:00",
    "now": "2025-03-08T10:15:00+00:00"
  },
  "expected_outputs": {
    "delta_seconds": 181800,
    "remaining_days": 2,
    "remaining_hours": 2,
    "remaining_minutes": 30,
    "remaining_seconds": 0
  },
  "working": "delta = target − now = 2025-03-10 12:45 − 2025-03-08 10:15. From March 8 10:15 to March 10 10:15 = 2 days = 2 × 86400 = 172800 seconds. From 10:15 to 12:45 = 2 hours 30 minutes = (2 × 3600) + (30 × 60) = 7200 + 1800 = 9000 seconds. Total delta = 172800 + 9000 = 181800 seconds = 2 days, 2 hours, 30 minutes, 0 seconds."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

The timezone-less target produces different valid countdowns under plausible conventions: 24 hours when interpreted as UTC versus 28 hours when interpreted as America/New_York. The instrument should require or explicitly document timezone handling.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche5/tranche5-results.json
• BOUNDARY_HAZARD_CATALOG.json