Cameron Cordes
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- CONTEXT.md: date/time vocabulary (true instant, wall clock, camera offset, display zone, filename date class, ...) - ADR-0001: dates are true instants; wall clocks are a display concern - un-ignore docs/adr/ so decisions land in git
62 lines
3.7 KiB
Markdown
62 lines
3.7 KiB
Markdown
# ADR-0001: Dates are true instants; wall clocks are a display concern
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**Status**: accepted (2026-09-24)
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## Context
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`image_exif.date_taken` (Unix i64) accumulated four conventions in one column, indistinguishable
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without joining on `date_taken_source`: EXIF rows were camera wall-clocks reinterpreted as UTC,
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exiftool rows were naive dates interpreted in the server's local zone, filename rows were
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deliberate wall-as-UTC, and filesystem-time rows were true instants. SMS-API and the Apollo
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location track were already true UTC. Every consumer had to re-derive the missing shift
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(Apollo's `_naive_to_real_utc`, the frontend `naiveAsUtcToRealUtc`, FileViewer's bare
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device-local formatting, `/memories`'s per-client offset) and they disagreed: the same photo
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showed different clocks in the two apps, exiftool-sourced videos (whose dates only come from
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exiftool) were double-shifted, and the insight-context prompt formatted a value 4–5 hours
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wrong for video rows, rolling across midnight near day edges.
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## Decision
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The date a media file was captured is stored as a **true instant** (Unix seconds of the
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actual moment). The wall clock the camera showed is a **display value**, recovered from the
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instant plus a per-row **camera offset** resolved at ingest by a cascade (explicit EXIF
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offset tag → GPS position → server zone, the last recorded as an assumption). Day-level
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grouping uses the owner's **display zone** (an IANA zone name). Manual edits enter as wall
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clocks in the display zone and are stored as instants.
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Consequences of the decision:
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- Matching photo↔SMS, photo↔location, and photo↔calendar becomes instant-to-instant with no
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per-consumer shift; the re-anchoring helpers in the consumers are deleted, not migrated.
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- A photo's displayed time can differ from the owner's local time (a photo taken in Japan
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shows the Japan wall clock); grouping still uses the owner's day. This is deliberate.
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- The legacy `date_taken` column is frozen during migration (a new `date_utc` column is
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populated by a per-tick backfill drain), so rollback is a client-side flip and the old
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contract stays byte-identical until cutover. The legacy column is retained after cutover
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for historical context.
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## Considered options
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- **Standardize on wall-clock-as-UTC** (fix only the exiftool branch): cheapest, but every
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consumer would still carry a display-zone re-anchor as a proxy for the camera's zone, and
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cross-zone photo↔SMS matching would stay approximate. Rejected.
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- **Two parallel fields as a permanent contract** (wall-as-UTC `date_taken` + `date_utc`):
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honest, but leaves the ambiguity in the interface forever. The dual-field shape is used
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*temporarily* (migration window) only.
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- **Server-side conversion per request** (keep the column as-is, convert on read): pushes
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the mixed-convention join onto every query and every consumer; rejected.
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## Non-obvious rules worth remembering
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- **Filename date class outranks EXIF** — for files whose filename carries the capture date
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(screenshots, timestamp exports), the filename is the original date of record, inverting
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the global EXIF-wins rule. The class is a documented exception, not a bug.
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- **Offset resolution is per-row and audited** (`date_taken_tz_source`), because a library
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contains photos from many zones; a single global offset would be wrong for every trip.
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- **The display zone is a zone name, not minutes** — a hardcoded offset silently breaks at
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DST. (The legacy `tz_offset_minutes` default of -480 on a New York server is a surviving
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symptom of this.)
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- **Re-dating across a day boundary invalidates derived day data** (daily summaries, reel
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pre-gen), which regenerates on its normal schedule; the backfill drain detects the day
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change per row.
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