npm install date-fns

import {
  format,
  parseISO,
  addDays,
  differenceInDays,
  formatDistance,
  isAfter,
  startOfWeek,
  endOfWeek,
} from 'date-fns';

// Format a date for display
const now = new Date();
console.log(format(now, 'MMMM do, yyyy'));        // "April 14th, 2026"
console.log(format(now, 'yyyy-MM-dd HH:mm:ss'));  // "2026-04-14 09:30:00"

// Parse an ISO string from an API response
const dueDate = parseISO('2026-05-01T00:00:00Z');
console.log(differenceInDays(dueDate, now));       // days until due

// Human-readable relative time
const postedAt = parseISO('2026-04-10T14:00:00Z');
console.log(formatDistance(postedAt, now, { addSuffix: true }));
// "4 days ago"

// Add time to a date (returns new Date, no mutation)
const nextWeek = addDays(now, 7);
console.log(isAfter(nextWeek, now)); // true

// Get the start and end of the current week (for calendar views)
const weekStart = startOfWeek(now, { weekStartsOn: 1 }); // Monday
const weekEnd = endOfWeek(now, { weekStartsOn: 1 });

// Locale-aware formatting
import { enUS, fr, ja } from 'date-fns/locale';
console.log(format(now, 'PPPP', { locale: fr }));
// "lundi 14 avril 2026"