ImageApi/docs/adr/0001-true-utc-dates.md

ADR-0001: Dates are true instants; wall clocks are a display concern

Status: accepted (2026-09-24)

Context

image_exif.date_taken (Unix i64) accumulated four conventions in one column, indistinguishable without joining on date_taken_source: EXIF rows were camera wall-clocks reinterpreted as UTC, exiftool rows were naive dates interpreted in the server's local zone, filename rows were deliberate wall-as-UTC, and filesystem-time rows were true instants. SMS-API and the Apollo location track were already true UTC. Every consumer had to re-derive the missing shift (Apollo's _naive_to_real_utc, the frontend naiveAsUtcToRealUtc, FileViewer's bare device-local formatting, /memories's per-client offset) and they disagreed: the same photo showed different clocks in the two apps, exiftool-sourced videos (whose dates only come from exiftool) were double-shifted, and the insight-context prompt formatted a value 4–5 hours wrong for video rows, rolling across midnight near day edges.

Decision

The date a media file was captured is stored as a true instant (Unix seconds of the actual moment). The wall clock the camera showed is a display value, recovered from the instant plus a per-row camera offset resolved at ingest by a cascade (explicit EXIF offset tag → GPS position → server zone, the last recorded as an assumption). Day-level grouping uses the owner's display zone (an IANA zone name). Manual edits enter as wall clocks in the display zone and are stored as instants.

Consequences of the decision:

• Matching photo↔SMS, photo↔location, and photo↔calendar becomes instant-to-instant with no per-consumer shift; the re-anchoring helpers in the consumers are deleted, not migrated.
• A photo's displayed time can differ from the owner's local time (a photo taken in Japan shows the Japan wall clock); grouping still uses the owner's day. This is deliberate.
• The legacy date_taken column is frozen during migration (a new date_utc column is populated by a per-tick backfill drain), so rollback is a client-side flip and the old contract stays byte-identical until cutover. The legacy column is retained after cutover for historical context.

Considered options

• Standardize on wall-clock-as-UTC (fix only the exiftool branch): cheapest, but every consumer would still carry a display-zone re-anchor as a proxy for the camera's zone, and cross-zone photo↔SMS matching would stay approximate. Rejected.
• Two parallel fields as a permanent contract (wall-as-UTC date_taken + date_utc): honest, but leaves the ambiguity in the interface forever. The dual-field shape is used temporarily (migration window) only.
• Server-side conversion per request (keep the column as-is, convert on read): pushes the mixed-convention join onto every query and every consumer; rejected.

Non-obvious rules worth remembering

• Filename date class outranks EXIF — for files whose filename carries the capture date (screenshots, timestamp exports), the filename is the original date of record, inverting the global EXIF-wins rule. The class is a documented exception, not a bug.
• Offset resolution is per-row and audited (date_taken_tz_source), because a library contains photos from many zones; a single global offset would be wrong for every trip.
• The display zone is a zone name, not minutes — a hardcoded offset silently breaks at DST. (The legacy tz_offset_minutes default of -480 on a New York server is a surviving symptom of this.)
• Re-dating across a day boundary invalidates derived day data (daily summaries, reel pre-gen), which regenerates on its normal schedule; the backfill drain detects the day change per row.