At most one date library

Two date libraries mean two timezone-handling strategies, two parsing APIs, and two bundle payloads. date-fns treats dates as immutable UTC-first while dayjs mirrors Moment's mutable-looking API — code that mixes both produces DST-transition bugs that only fire twice a year. Formatting output drifts between components (one says "Jan 5" the other says "1/5"), and locale handling requires loading the right locale pack into the right library or date strings silently fall back to English.

Wrap your chosen library in src/utils/dates.ts and import only that wrapper from the rest of the codebase.

// src/utils/dates.ts
import { format, parseISO } from 'date-fns'
export const formatDate = (d: Date) => format(d, 'yyyy-MM-dd')
export const parseDate = (s: string) => parseISO(s)

Rewrite every dayjs call site, then npm uninstall dayjs. Add an ESLint rule forbidding direct imports of the removed library anywhere outside the wrapper module to prevent regression.

• ID: ai-slop-code-drift.tooling-stack-drift.dual-date-library

• Severity: medium

• What to look for: Apply the three-condition rule against this exact date library allowlist: date-fns, dayjs, luxon, moment, @js-joda/core, temporal-polyfill, js-time-ago, time-fns. Note: native Date and the upcoming Temporal API are NOT packages — do not count them. Count all REMAINING libraries that meet all three conditions (at least 1 non-escape-hatch importing file).

• Pass criteria: 0 or 1 date libraries from the allowlist actively used. Report even on pass: "Canonical date library: [name or 'native Date'] ([N] importing files)."

• Fail criteria: 2 or more date libraries from the allowlist meet all three conditions (at least 1 non-escape-hatch importing file).

• Skip (N/A) when: 0 date libraries from the allowlist appear in RUNTIME_DEPS.

• Detail on fail: "2 active date libraries: 'date-fns' (14 files) AND 'dayjs' (6 files). Pick one for date handling."

• Remediation: Two date libraries means two timezone behaviors, two formatting APIs, two parsing strategies — and bugs that only show up at DST transitions. Pick one and migrate:

  // Bad: src/utils/format-date.ts uses date-fns, src/utils/parse-date.ts uses dayjs
  // Good: a single src/utils/dates.ts that wraps your chosen library
  
  // src/utils/dates.ts
  import { format, parseISO } from 'date-fns'
  export const formatDate = (d: Date) => format(d, 'yyyy-MM-dd')