const d = new Date('2026-05-16');
// Works on your machine, fails on others
// Timezone issues everywhere
// Month is 0-indexed (January = 0)
// Year before 100 = 1900 + year

npm install dayjs  # 2KB, immutable, chainable
# or: npm install date-fns  # Tree-shakeable, functional

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

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

// Parse
dayjs('2026-05-16');                    // May 16, 2026
dayjs.unix(1715840000);                 // From Unix timestamp
dayjs();                                // Now

// Format
dayjs().format('YYYY-MM-DD');           // "2026-05-16"
dayjs().format('YYYY-MM-DD HH:mm:ss');  // "2026-05-16 07:20:00"
dayjs().format('ddd, MMM D, YYYY');     // "Sat, May 16, 2026"

// Manipulate
dayjs().add(7, 'day');                  // Next week
dayjs().subtract(1, 'month');           // Last month
dayjs().startOf('week');                // Monday of this week
dayjs().endOf('month');                // Last day of this month

// Query
dayjs().isBefore(dayjs('2027-01-01'));  // true
dayjs().isAfter(dayjs('2026-01-01'));   // true
dayjs().isSame(dayjs(), 'day');         // true

// Relative time
dayjs().fromNow();                      // "a few seconds ago"
dayjs('2026-06-01').fromNow();          // "in 2 weeks"

// Timezone
dayjs().tz('America/New_York').format(); // Eastern time
dayjs().tz('Asia/Shanghai').format();    // Beijing time
dayjs().utc().format();                  // UTC

// Difference
dayjs('2026-12-31').diff(dayjs(), 'day'); // Days until New Year's Eve

import { format, addDays, subMonths, isAfter, parseISO } from 'date-fns';

const now = new Date();
format(now, 'yyyy-MM-dd');              // "2026-05-16"
addDays(now, 30);                       // 30 days from now
subMonths(now, 3);                      // 3 months ago
isAfter(new Date('2027-01-01'), now);   // true
parseISO('2026-05-16T07:20:00Z');       // Date object

const now = new Date();

// Get components
now.getFullYear();      // 2026
now.getMonth();         // 4 (May! 0-indexed!)
now.getDate();          // 16
now.getDay();           // 6 (Saturday, 0=Sunday)
now.getHours();         // 7
now.getMinutes();       // 20
now.getSeconds();       // 0
now.getTime();          // Unix timestamp in ms

// Set components
const d = new Date();
d.setFullYear(2026);
d.setMonth(11);         // December (0-indexed!)
d.setDate(25);          // Christmas!

// Format (vanilla is ugly)
d.toISOString();        // "2026-12-25T00:00:00.000Z"
d.toLocaleDateString('en-US', { 
  weekday: 'long', 
  year: 'numeric', 
  month: 'long', 
  day: 'numeric' 
});                      // "Friday, December 25, 2026"

// Parse (careful with formats)
new Date('2026-05-16');            // OK (ISO format)
new Date('05/16/2026');             // Depends on locale!
new Date(2026, 4, 16);             // Reliable (month is 0-indexed!)

// Compare
const d1 = new Date('2026-01-01');
const d2 = new Date('2026-12-31');
d1 < d2;                           // true
Math.abs(d2 - d1) / (1000 * 60 * 60 * 24); // Days between dates

function formatDuration(ms) {
  const seconds = Math.floor(ms / 1000);
  const minutes = Math.floor(seconds / 60);
  const hours = Math.floor(minutes / 60);
  const days = Math.floor(hours / 24);

  if (days > 0) return `${days}d ${hours % 24}h`;
  if (hours > 0) return `${hours}h ${minutes % 60}m`;
  if (minutes > 0) return `${minutes}m ${seconds % 60}s`;
  return `${seconds}s`;
}

formatDuration(3661000);    // "1h 1m"
formatDuration(90061000);   // "1d 1h"

function isBetween(date, start, end) {
  const d = new Date(date).setHours(0, 0, 0, 0);
  const s = new Date(start).setHours(0, 0, 0, 0);
  const e = new Date(end).setHours(0, 0, 0, 0);
  return d >= s && d <= e;
}

isBetween(new Date(), '2026-05-01', '2026-05-31'); // true

function getDateRange(start, end) {
  const dates = [];
  let current = new Date(start);
  const stop = new Date(end);

  while (current <= stop) {
    dates.push(new Date(current));
    current.setDate(current.getDate() + 1);
  }

  return dates;
}

getDateRange('2026-05-01', '2026-05-05');
// [May 1, May 2, May 3, May 4, May 5]

function countdown(targetDate) {
  const update = () => {
    const now = new Date();
    const diff = new Date(targetDate) - now;

    if (diff <= 0) return { done: true };

    const days = Math.floor(diff / (1000 * 60 * 60 * 24));
    const hours = Math.floor((diff % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
    const minutes = Math.floor((diff % (1000 * 60 * 60)) / (1000 * 60));
    const seconds = Math.floor((diff % (1000 * 60)) / 1000);

    return { days, hours, minutes, seconds };
  };

  return update();
}

// ❌ Don't do this:
new Date().toString(); // "Sat May 16 2026 07:20:00 GMT+0800..."

// ✅ Do this instead:
new Date().toISOString(); // Always UTC
Intl.DateTimeFormat('en-US', { timeZone: 'America/New_York' }).format(now);

// Get user's timezone
Intl.DateTimeFormat().resolvedOptions().timeZone; // "Asia/Shanghai"

// Convert timezone
function convertTZ(date, tz) {
  return new Date(date.toLocaleString('en-US', { timeZone: tz }));
}