Working with Dates and Timestamps in JavaScript — Complete Guide

JavaScript date handling has a reputation for being confusing. Between the Date object, Unix timestamps, timezones, and formatting, there's a lot to know. This guide covers every operation you'll actually need.

Getting the current date and time

const now = new Date();

now.getTime()           // Unix timestamp in milliseconds: 1700000000000
Math.floor(now.getTime() / 1000) // Unix timestamp in seconds: 1700000000
now.toISOString()       // "2023-11-14T22:13:20.000Z"
now.toLocaleDateString() // "11/14/2023" (locale-dependent)
now.toLocaleString()    // "11/14/2023, 10:13:20 PM"
now.toString()          // "Tue Nov 14 2023 22:13:20 GMT+0000"

Creating a Date from a timestamp

// From Unix timestamp (seconds)
const date = new Date(1700000000 * 1000); // multiply by 1000

// From Unix timestamp (milliseconds)
const date = new Date(1700000000000);

// From ISO string
const date = new Date('2023-11-14T22:13:20Z');

// From date parts (CAREFUL: month is 0-indexed!)
const date = new Date(2023, 10, 14); // November 14 (month 10 = November)

Formatting dates

const date = new Date('2023-11-14T22:13:20Z');

// Intl.DateTimeFormat (recommended)
new Intl.DateTimeFormat('en-US').format(date)
// "11/14/2023"

new Intl.DateTimeFormat('en-GB').format(date)
// "14/11/2023"

new Intl.DateTimeFormat('en-US', {
  year: 'numeric', month: 'long', day: 'numeric'
}).format(date)
// "November 14, 2023"

new Intl.DateTimeFormat('en-US', {
  dateStyle: 'full', timeStyle: 'short'
}).format(date)
// "Tuesday, November 14, 2023 at 10:13 PM"

// Manual formatting
const pad = n => String(n).padStart(2, '0');
const d = new Date();
const formatted = d.getFullYear() + '-' + pad(d.getMonth()+1) + '-' + pad(d.getDate());
// "2023-11-14"

Date arithmetic

const now = new Date();

// Add 7 days
const nextWeek = new Date(now.getTime() + 7 * 24 * 60 * 60 * 1000);

// Add 1 month (careful with month-end edge cases)
const nextMonth = new Date(now);
nextMonth.setMonth(nextMonth.getMonth() + 1);

// Difference between two dates
const start = new Date('2023-01-01');
const end = new Date('2023-12-31');
const diffMs = end - start;
const diffDays = diffMs / (1000 * 60 * 60 * 24); // 364 days

// Is date in the past?
const isPast = date < new Date();

// Is date today?
const isToday = date.toDateString() === new Date().toDateString();

Timezone handling

// All Date objects are stored internally in UTC
const date = new Date('2023-11-14T22:13:20Z'); // Z = UTC

// Get UTC values
date.getUTCHours()    // 22
date.getUTCDate()     // 14

// Get local values (depends on system timezone)
date.getHours()       // varies by timezone
date.getDate()        // may differ from UTC date

// Format in specific timezone
new Intl.DateTimeFormat('en-US', {
  timeZone: 'America/New_York',
  dateStyle: 'short',
  timeStyle: 'short'
}).format(date)

// Get timezone offset
date.getTimezoneOffset() // minutes offset from UTC (negative = east of UTC)

Common gotchas

• Months are 0-indexed: January = 0, December = 11
• new Date("2023-11-14") is UTC, new Date("11/14/2023") is local time — this trips everyone up
• Date comparison: Use .getTime() for reliable comparison, not ==
• Invalid dates: Always check isNaN(date.getTime()) after parsing user input