`instant.toISOString().slice(0, 10)` and parsing a key back as UTC
midnight are the idioms behind every day-boundary fix this year: the
evening west of UTC and the small hours east of it land on the
neighbouring day. A new rule, healthlog/no-utc-day-key, refuses them in
src/ outside src/lib/tz and tests, and refuses a disable directive for
itself that is not next-line or gives no reason.
The time zone module now names every legitimate form: userDayKey for a
person's day, dateOnlyKey / dateOnlyAtNoonUtc / dayKeyAsUtcMidnight for
date-only values and day labels, daysBetweenDateKeys / weekdayOfDateKey /
isCalendarDateKey / statedDateKey for key arithmetic and parsing.
Of the 133 sites the rule found, 84 are gone. Most moved onto a named
helper with no change in behaviour; these changed behaviour:
- cycle calendar, insights, Coach snapshot and prediction refresh start
their lookbacks at the first instant of the user's day
- a model-transcribed date like "June 10, 2026" no longer lands a day
early in a process east of UTC (documents and lab scans)
- a custom report range covers exactly the picked days in the profile
zone instead of starting the evening before west of UTC
- date-of-birth pickers cap at the user's today, not the UTC one
- an allergy onset date is stored at noon UTC like other date-only values
- the report's cycle summary, the Coach's usual BP range and the intake
projector's rollup recompute use the user's day
- vaccination dates render as dates instead of the previous evening
49 remain, each with its reason on the line above: 39 UTC by design (15
file name stamps, rollup and object keys, the UTC-day score engines) and
10 marked baseline. A guard test requires a classified reason on every
directive and holds the baseline to a ceiling that only goes down.
One-shot prettier pass so the new format:check gate starts from a clean
baseline. Formatting only — no behaviour change. Generator-owned and
vendored artifacts stay untouched via .prettierignore.
Add a short-horizon OLS projection over the DAY-native per-day mean
series, extending the existing trendSlope. The fit, residual standard
error and prediction interval are computed at DAY granularity only — the
rollup invariant forbids composing a slope/sd from WEEK/MONTH buckets.
The projection is gated on fit quality (R squared), in-window history and
staleness: below any floor the engine returns insufficient rather than a
weak line, so noise is never extrapolated. The prediction-interval band
widens with the horizon distance from the data centre, so the fan
visibly communicates uncertainty.
Register TRAJECTORY in the derived registry and dispatch (additive,
iOS-forward-compatible; the capabilities endpoint and drift-guard pick it
up automatically). Surface it on the weight detail page as a forecast
band beneath the chart, labelled a projection with the cited method and
a conditional, range-only headline — never a prediction or a dated
event. i18n across all six locales.
Marc-André Bombeck