useDate

interface DateAdapter<T> {
  locale?: string
  /** First day of week: 0 = Sunday, 1 = Monday, … 6 = Saturday. Set by the plugin from locale. */
  firstDayOfWeek?: number

  // Construction & Conversion
  date (value?: unknown): T | null
  toJsDate (value: T): Date
  parseISO (dateString: string): T
  toISO (date: T): string
  parse (value: string, format: string): T | null
  isValid (date: unknown): date is T  // Type predicate
  isNull (value: T | null): value is null  // Type predicate

  // Formatting
  format (date: T, formatString: string): string
  formatByString (date: T, formatString: string): string
  formatNumber (numberToFormat: string): string
  getFormatHelperText (format: string): string
  getMeridiemText (ampm: 'am' | 'pm'): string

  // Navigation
  startOfDay (date: T): T
  endOfDay (date: T): T
  startOfWeek (date: T, firstDayOfWeek?: number): T
  endOfWeek (date: T, firstDayOfWeek?: number): T
  startOfMonth (date: T): T
  endOfMonth (date: T): T
  startOfYear (date: T): T
  endOfYear (date: T): T

  // Arithmetic
  addSeconds (date: T, amount: number): T
  addMinutes (date: T, amount: number): T
  addHours (date: T, amount: number): T
  addDays (date: T, amount: number): T
  addWeeks (date: T, amount: number): T
  addMonths (date: T, amount: number): T
  addYears (date: T, amount: number): T

  // Comparison
  isAfter (date: T, comparing: T): boolean
  isAfterDay (date: T, comparing: T): boolean
  isAfterMonth (date: T, comparing: T): boolean
  isAfterYear (date: T, comparing: T): boolean
  isBefore (date: T, comparing: T): boolean
  isBeforeDay (date: T, comparing: T): boolean
  isBeforeMonth (date: T, comparing: T): boolean
  isBeforeYear (date: T, comparing: T): boolean
  isEqual (date: T, comparing: T): boolean
  isSameDay (date: T, comparing: T): boolean
  isSameHour (date: T, comparing: T): boolean
  isSameMonth (date: T, comparing: T): boolean
  isSameYear (date: T, comparing: T): boolean
  isWithinRange (date: T, range: [T, T]): boolean

  // Getters
  getYear (date: T): number
  getMonth (date: T): number  // 0-indexed
  getDate (date: T): number
  getHours (date: T): number
  getMinutes (date: T): number
  getSeconds (date: T): number
  getDiff (date: T, comparing: T | string, unit?: string): number
  getWeek (date: T, firstDayOfWeek?: number, minimalDays?: number): number
  getDaysInMonth (date: T): number

  // Setters (immutable - returns new instance)
  setYear (date: T, year: number): T
  setMonth (date: T, month: number): T  // 0-indexed
  setDate (date: T, day: number): T
  setHours (date: T, hours: number): T
  setMinutes (date: T, minutes: number): T
  setSeconds (date: T, seconds: number): T

  // Calendar Utilities
  getWeekdays (format?: 'long' | 'short' | 'narrow'): string[]
  getWeekArray (date: T, firstDayOfWeek?: number): T[][]
  getMonthArray (date: T): T[]
  getYearRange (start: T, end: T): T[]
  getNextMonth (date: T): T
  getPreviousMonth (date: T): T

  // Utility
  mergeDateAndTime (date: T, time: T): T
  getCurrentLocaleCode (): string
  is12HourCycleInCurrentLocale (): boolean
}

import type { DateAdapter } from '@vuetify/v0'
import { isValid as dateFnsIsValid, parseISO, format as dateFnsFormat } from 'date-fns'

class DateFnsAdapter implements DateAdapter<Date> {
  locale = 'en-US'

  date (value?: unknown): Date | null {
    if (value == null) return new Date()
    if (value instanceof Date) return value
    if (typeof value === 'string') return parseISO(value)
    if (typeof value === 'number') return new Date(value)
    return null
  }

  // Type predicate - enables TypeScript narrowing
  isValid (date: unknown): date is Date {
    return date instanceof Date && dateFnsIsValid(date)
  }

  // Type predicate - enables TypeScript narrowing
  isNull (value: Date | null): value is null {
    return value === null
  }

  format (date: Date, formatString: string): string {
    return dateFnsFormat(date, this.getDateFnsFormat(formatString))
  }

  // Implement remaining methods...
}

// Use with plugin
app.use(
  createDatePlugin({
    adapter: new DateFnsAdapter(),
  })
)