Day.js

import dayjs from 'dayjs';

const dayjs = require('dayjs');

import dayjs from 'dayjs';
import utc from 'dayjs/plugin/utc';
import timezone from 'dayjs/plugin/timezone';

dayjs.extend(utc);
dayjs.extend(timezone);

import dayjs from 'dayjs';

// Create dayjs objects
const now = dayjs();
const birthday = dayjs('1990-01-25');
const timestamp = dayjs(1318781876406);

// Chainable manipulation
const nextWeek = dayjs()
  .add(1, 'week')
  .startOf('day')
  .format('YYYY-MM-DD');

// Query operations
if (dayjs().isAfter(birthday)) {
  console.log('Birthday has passed');
}

// Format display
console.log(dayjs().format('YYYY-MM-DD HH:mm:ss'));
console.log(dayjs().format('[Today is] dddd'));

/**
 * Creates a new Dayjs instance (basic overload)
 * @param date - Input date (string, number, Date, Dayjs, null, undefined)
 * @returns New Dayjs instance
 */
function dayjs(date?: ConfigType): Dayjs;

/**
 * Creates a new Dayjs instance with format (format overload)
 * @param date - Input date
 * @param format - Parse format string or format object
 * @param strict - Strict parsing mode
 * @returns New Dayjs instance
 */
function dayjs(date?: ConfigType, format?: OptionType, strict?: boolean): Dayjs;

/**
 * Creates a new Dayjs instance with format and locale (full overload)
 * @param date - Input date
 * @param format - Parse format string or format object
 * @param locale - Locale string
 * @param strict - Strict parsing mode
 * @returns New Dayjs instance
 */
function dayjs(date?: ConfigType, format?: OptionType, locale?: string, strict?: boolean): Dayjs;

type ConfigType = string | number | Date | Dayjs | null | undefined;
type OptionType = FormatObject | string | string[];

interface FormatObject {
  locale?: string;
  format?: string;
  utc?: boolean;
}

/**
 * Extend dayjs with plugin functionality
 * @param plugin - Plugin function
 * @param option - Plugin options
 * @returns dayjs function for chaining
 */
dayjs.extend<T>(plugin: PluginFunc<T>, option?: T): typeof dayjs;

/**
 * Set or get global locale
 * @param preset - Locale name or locale object
 * @param object - Locale definition object
 * @param isLocal - Local-only setting
 * @returns Current locale name or dayjs function
 */
dayjs.locale(preset?: string | ILocale, object?: Partial<ILocale>, isLocal?: boolean): string;

/**
 * Check if value is a Dayjs instance
 * @param d - Value to check
 * @returns True if value is Dayjs instance
 */
dayjs.isDayjs(d: any): d is Dayjs;

/**
 * Create Dayjs from Unix timestamp
 * @param timestamp - Unix timestamp in seconds
 * @returns New Dayjs instance
 */
dayjs.unix(timestamp: number): Dayjs;

interface PluginFunc<T = unknown> {
  (option: T, dayjsClass: typeof Dayjs, dayjsFactory: typeof dayjs): void;
}

// Plugin installation
dayjs.extend(pluginFunction, options);

// Global locale setting
dayjs.locale('es'); // Spanish globally

// Instance-specific locale
dayjs().locale('zh-cn'); // Chinese for this instance

// Locale object structure
interface ILocale {
  name: string;
  weekdays?: string[];
  months?: string[];
  weekStart?: number;
  yearStart?: number;
  ordinal?: (n: number) => string;
  formats?: Record<string, string>;
  relativeTime?: Record<string, string>;
}

// Main Dayjs class interface
interface Dayjs {
  // Core methods - see Core API documentation for full details
  year(): number;
  year(value: number): Dayjs;
  month(): number;
  month(value: number): Dayjs;
  date(): number;
  date(value: number): Dayjs;
  day(): 0 | 1 | 2 | 3 | 4 | 5 | 6;
  day(value: number): Dayjs;
  hour(): number;
  hour(value: number): Dayjs;
  minute(): number;
  minute(value: number): Dayjs;
  second(): number;
  second(value: number): Dayjs;
  millisecond(): number;
  millisecond(value: number): Dayjs;
  
  set(unit: UnitType, value: number): Dayjs;
  get(unit: UnitType): number;
  add(value: number, unit?: ManipulateType): Dayjs;
  subtract(value: number, unit?: ManipulateType): Dayjs;
  startOf(unit: OpUnitType): Dayjs;
  endOf(unit: OpUnitType): Dayjs;
  
  format(template?: string): string;
  diff(date?: ConfigType, unit?: QUnitType | OpUnitType, float?: boolean): number;
  valueOf(): number;
  unix(): number;
  daysInMonth(): number;
  toDate(): Date;
  toJSON(): string;
  toISOString(): string;
  toString(): string;
  utcOffset(): number;
  
  isBefore(date?: ConfigType, unit?: OpUnitType): boolean;
  isSame(date?: ConfigType, unit?: OpUnitType): boolean;
  isAfter(date?: ConfigType, unit?: OpUnitType): boolean;
  isValid(): boolean;
  
  clone(): Dayjs;
  locale(): string;
  locale(preset: string | ILocale, object?: Partial<ILocale>): Dayjs;
}

// Type definitions for units and configuration
type ConfigType = string | number | Date | Dayjs | null | undefined;

type UnitTypeShort = 'd' | 'D' | 'M' | 'y' | 'h' | 'm' | 's' | 'ms';
type UnitTypeLong = 'millisecond' | 'second' | 'minute' | 'hour' | 'day' | 'month' | 'year' | 'date';
type UnitTypeLongPlural = 'milliseconds' | 'seconds' | 'minutes' | 'hours' | 'days' | 'months' | 'years' | 'dates';
type UnitType = UnitTypeLong | UnitTypeLongPlural | UnitTypeShort;

type OpUnitType = UnitType | "week" | "weeks" | 'w';
type QUnitType = UnitType | "quarter" | "quarters" | 'Q';
type ManipulateType = Exclude<OpUnitType, 'date' | 'dates'>;