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.