JavaScript - Date & Math

Overview

Estimated time: 35–45 minutes

Learn how to create and format dates, perform date arithmetic, handle time zones, and use common Math utilities for rounding and random numbers.

Learning Objectives

• Create Date objects safely and format them.
• Compute differences, add/subtract days, and handle UTC vs local time.
• Use Math for rounding, clamping, and random ranges.

Prerequisites

• JavaScript - Numbers & BigInt
• JavaScript - RegExp (recommended)

Creating dates

// Now
const now = new Date();               // local time

// From timestamp (ms since Unix epoch)
const t = Date.now();                 // number
const fromTs = new Date(t);           // Date

// From ISO 8601 string (portable)
const iso = new Date('2025-09-05T12:00:00Z'); // UTC

// Components: months are 0-based (0=Jan, 11=Dec)
const local = new Date(2025, 8, 5, 9, 30); // 2025-09-05 09:30 local

// UTC factory to avoid DST surprises when building calendar dates
const utc = new Date(Date.UTC(2025, 8, 5, 9, 30));

Formatting and parts

now.toISOString();          // '2025-09-05T14:22:31.123Z'
now.getFullYear();          // local year
now.getUTCFullYear();       // UTC year
now.getTimezoneOffset();    // minutes difference from UTC

Date arithmetic

// Difference in days
const a = new Date('2025-09-01T00:00:00Z');
const b = new Date('2025-09-05T00:00:00Z');
const diffMs = b - a;                   // number (ms)
const diffDays = diffMs / (1000*60*60*24); // 4

// Add days (UTC-safe approach)
function addDaysUTC(d, days) {
  const nd = new Date(d.getTime());
  nd.setUTCDate(nd.getUTCDate() + days);
  return nd;
}

addDaysUTC(new Date('2025-09-05T00:00:00Z'), 7).toISOString();
// '2025-09-12T00:00:00.000Z'

Math utilities

// Rounding
Math.floor(3.7);  // 3
Math.ceil(3.1);   // 4
Math.round(3.5);  // 4
Math.trunc(-3.9); // -3

// Clamp
const clamp = (x, lo, hi) => Math.min(hi, Math.max(lo, x));
clamp(15, 0, 10); // 10

// Random in range [min, max) for numbers
function randRange(min, max) {
  return Math.random() * (max - min) + min;
}

// Random integer in [min, max] inclusive
function randInt(min, max) {
  min = Math.ceil(min); max = Math.floor(max);
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

Common Pitfalls

• Months are 0-based: September is 8, not 9.
• DST shifts: Adding hours across DST boundaries can skip/repeat times; prefer UTC arithmetic for calendar math.
• Parsing: Non-ISO date strings are locale-dependent. Prefer ISO 8601 or numeric constructors.
• Time zones: Date stores a UTC timestamp; formatting decides local vs UTC.
• Random: Math.random() is not cryptographically secure; use crypto.getRandomValues when needed.

Try it