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Numbers, Math and dates: money, rounding, random numbers and time

Almost every real app calculates: cart totals with VAT, loan interest, exam averages, discounts, random OTP codes, countdowns, "posted 3 hours ago", and due dates. JavaScript's numbers, the Math object and Date (plus the newer Intl formatting tools) handle all of this, but each has traps: floating-point errors with money, months that start at 0, and time zones. This unit teaches the right way.

Numbers in JavaScript

JavaScript has one main number type (64-bit floating point) for integers and decimals: 42, 3.14, -7. (There's also BigInt for huge integers, rarely needed.)

JavaScript · runs live in the interactive lesson
console.log(typeof 42, typeof 3.5);    // number number
console.log(10 / 3);                   // 3.3333333333333335
console.log(1 / 0);                    // Infinity
console.log("abc" * 2);                // NaN (Not a Number)
console.log(Number.isNaN("abc" * 2));  // true
console.log(Number.MAX_SAFE_INTEGER);  // 9007199254740991

Converting strings to numbers

Form inputs and URL parameters are strings. Convert them before calculating:

JavaScript · runs live in the interactive lesson
console.log("100" + 50);               // "10050" (string joining!)
console.log(Number("100") + 50);       // 150
console.log(parseInt("250 bob", 10));  // 250 (reads digits at the start)
console.log(parseFloat("3.75kg"));     // 3.75
console.log(Number(""));               // 0 (careful: empty becomes 0)
console.log(Number("12a"));            // NaN
console.log(+"42");                    // 42 (unary plus)

function readAmount(text) {
  const n = Number(String(text).replace(/,/g, ""));   // allow "1,500"
  return Number.isFinite(n) ? n : null;
}
console.log(readAmount("1,500"), readAmount("abc"));

Floating-point and money

Computers store decimals in binary, so some decimals can't be represented exactly:

JavaScript · runs live in the interactive lesson
console.log(0.1 + 0.2);                // 0.30000000000000004
console.log(0.1 + 0.2 === 0.3);        // false!

For money this matters. Safe approaches:

  1. Work in whole units. KSh amounts are usually whole shillings; for cents, store cents as integers (KSh 199.50 → 19950).
  2. Round at the end for display.
  3. For complex financial systems, use decimal libraries.
JavaScript · runs live in the interactive lesson
// Prices in cents (integers): exact maths
const items = [{ price: 19950, qty: 2 }, { price: 4999, qty: 3 }];   // KSh 199.50, KSh 49.99
const totalCents = items.reduce((s, i) => s + i.price * i.qty, 0);
console.log("Total:", (totalCents / 100).toFixed(2));                // "548.97"

// VAT included in a price (Kenya 16%): VAT part = price × 16 / 116
const price = 1160;
const vat = Math.round(price * 16 / 116);
console.log(`Price ${price} includes VAT of ${vat} (net ${price - vat})`);

Rounding

JavaScript · runs live in the interactive lesson
console.log(Math.round(4.5));      // 5   nearest integer (.5 rounds up)
console.log(Math.round(-4.5));     // -4  (rounds towards +infinity at .5)
console.log(Math.floor(4.9));      // 4   down
console.log(Math.ceil(4.1));       // 5   up
console.log(Math.trunc(-4.9));     // -4  remove decimals
console.log((3.14159).toFixed(2)); // "3.14" (a string)
console.log(Number((2.345).toFixed(2)));   // 2.35 or 2.34? floating point can surprise: test!

// Round to 2 decimals as a number
const round2 = (n) => Math.round((n + Number.EPSILON) * 100) / 100;
console.log(round2(1.005), round2(2.675));

Use ceil for "how many buses for 130 people at 33 seats each" (you can't hire a partial bus): Math.ceil(130 / 33) = 4.

The Math object

JavaScript · runs live in the interactive lesson
const sales = [3400, 12000, 7800, 15200];
console.log(Math.max(...sales), Math.min(...sales));   // 15200 3400
console.log(Math.abs(-250));       // 250
console.log(Math.pow(2, 10), 2 ** 10);   // 1024 1024
console.log(Math.sqrt(144));       // 12
console.log(Math.PI.toFixed(4));   // 3.1416

// Compound interest: amount = P × (1 + r)^n
const P = 50000, r = 0.12 / 12, n = 12;   // 12% a year, monthly, 1 year
console.log("After 1 year:", Math.round(P * (1 + r) ** n));

Random numbers

Math.random() gives a decimal from 0 (inclusive) up to 1 (exclusive):

JavaScript · runs live in the interactive lesson
const randomInt = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;

console.log(randomInt(1, 6));                     // dice roll
const towns = ["Nairobi", "Mombasa", "Kisumu", "Nakuru", "Eldoret"];
console.log(towns[randomInt(0, towns.length - 1)]);   // random pick

// Shuffle (Fisher–Yates) for quiz questions
const qs = [1, 2, 3, 4, 5];
for (let i = qs.length - 1; i > 0; i--) {
  const j = randomInt(0, i);
  [qs[i], qs[j]] = [qs[j], qs[i]];
}
console.log(qs);
JavaScript · runs live in the interactive lesson
const buf = new Uint32Array(1);
crypto.getRandomValues(buf);
const code = String(buf[0] % 1000000).padStart(6, "0");
console.log("Demo 6-digit code:", code);   // in real apps, generate on the server

Formatting numbers and currency

JavaScript · runs live in the interactive lesson
const n = 1234567.891;
console.log(n.toLocaleString("en-KE"));                               // "1,234,567.891"
console.log(n.toLocaleString("en-KE", { maximumFractionDigits: 0 })); // "1,234,568"

const ksh = new Intl.NumberFormat("en-KE", { style: "currency", currency: "KES" });
console.log(ksh.format(2500));             // e.g. "Ksh 2,500.00" (format depends on the browser/runtime data)

const compact = new Intl.NumberFormat("en", { notation: "compact" });
console.log(compact.format(1500000));      // "1.5M"

const pct = new Intl.NumberFormat("en", { style: "percent", maximumFractionDigits: 1 });
console.log(pct.format(0.163));            // "16.3%"

Many Kenyan sites prefer to write "KSh " + amount.toLocaleString("en-KE") for a consistent look.

Dates and times

Creating dates

JavaScript · runs live in the interactive lesson
const now = new Date();                                  // current date and time
const d1 = new Date("2026-10-02");                       // ISO date (treated as UTC midnight)
const d2 = new Date(2026, 9, 2, 14, 30);                 // year, MONTH (0-11!), day, hour, minute: local time
console.log(d2.getFullYear(), d2.getMonth(), d2.getDate()); // 2026 9 2  (9 = October)
console.log(d2.getDay());                                // day of week: 0 = Sunday
console.log(typeof now.getTime(), now.getTime() > 0);    // milliseconds since 1 Jan 1970 (UTC)

Adding days and calculating differences

JavaScript · runs live in the interactive lesson
const start = new Date(2026, 9, 2);           // 2 Oct 2026
const due = new Date(start);
due.setDate(due.getDate() + 30);              // add 30 days (month rollover handled)
console.log(due.toDateString());              // Sun Nov 01 2026

const DAY = 24 * 60 * 60 * 1000;
const exam = new Date(2026, 10, 3);           // 3 Nov 2026
const daysLeft = Math.ceil((exam - start) / DAY);
console.log(daysLeft, "days until the exam");

// Age from date of birth
function age(dob, today = new Date(2026, 9, 2)) {
  let a = today.getFullYear() - dob.getFullYear();
  const beforeBirthday = today.getMonth() < dob.getMonth() || (today.getMonth() === dob.getMonth() && today.getDate() < dob.getDate());
  return beforeBirthday ? a - 1 : a;
}
console.log(age(new Date(2005, 11, 15)));     // 20

Formatting dates

JavaScript · runs live in the interactive lesson
const d = new Date(Date.UTC(2026, 9, 2, 11, 30));   // 11:30 UTC = 14:30 in Nairobi (UTC+3)
const fmt = new Intl.DateTimeFormat("en-KE", { dateStyle: "full", timeStyle: "short", timeZone: "Africa/Nairobi" });
console.log(fmt.format(d));
console.log(d.toISOString());                        // "2026-10-02T11:30:00.000Z": for APIs and databases
console.log(d.toLocaleDateString("en-GB", { day: "numeric", month: "short", year: "numeric", timeZone: "Africa/Nairobi" }));

// "Time ago"
const rtf = new Intl.RelativeTimeFormat("en", { numeric: "auto" });
console.log(rtf.format(-3, "hour"), "|", rtf.format(1, "day"));

Time zones

  • Kenya uses East Africa Time (EAT), UTC+3, with no daylight saving time.
  • Computers store moments as UTC timestamps; display converts to the user's zone.
  • Store and send dates in ISO format with time zone (2026-10-02T14:30:00+03:00 or UTC with Z); never store "02/10/2026" text (is that 2 October or 10 February?).
  • For heavy date work, libraries like date-fns or Day.js help, and the upcoming Temporal API improves dates in JavaScript.

Countdown example

JavaScript · runs live in the interactive lesson
function countdown(target, now) {
  let ms = Math.max(0, target - now);
  const d = Math.floor(ms / 86400000); ms %= 86400000;
  const h = Math.floor(ms / 3600000);  ms %= 3600000;
  const m = Math.floor(ms / 60000);
  return `${d} days ${h} hours ${m} minutes`;
}
console.log(countdown(new Date(2026, 11, 25, 0, 0), new Date(2026, 9, 2, 9, 15)), "until Christmas");

Common mistakes

MistakeFix
"100" + 50 = "10050"Convert with Number() first
Comparing decimals with ===Use integers (cents) or a tolerance
toFixed returns a stringWrap in Number() if you need a number
Math.random() for OTPs/tokenscrypto.getRandomValues() / server-side
Months from 1new Date(2026, 0, 15) is 15 January
Storing local date stringsISO strings with time zone

Practice tasks

  1. Write addVat(net) and vatFromGross(gross) for 16% VAT, keeping whole shillings.
  2. Calculate a monthly loan repayment using the amortisation formula P × r / (1 − (1 + r)^−n).
  3. Generate a random quiz order of 10 questions with Fisher–Yates.
  4. Show "Due in X days" or "Overdue by X days" for an invoice date.
  5. Format 1234567 as KSh currency and as "1.2M".

Summary

  • JavaScript numbers are floating point; convert strings with Number, parseInt, parseFloat; check Number.isFinite.
  • Handle money in whole units or cents to avoid floating-point errors; round for display.
  • Math gives rounding, max/min, powers, roots and random (never for security; use crypto).
  • Format with toLocaleString and Intl.NumberFormat.
  • Dates: months are 0–11; add days with setDate; differences in milliseconds; format with Intl.DateTimeFormat; Kenya is UTC+3; store ISO strings.

Check yourself

  1. What does "5" + 5 give in JavaScript?

    Show answer

    55

  2. What does 0.1 + 0.2 === 0.3 give?

    Show answer

    false

  3. Which Math method always rounds down?

    Show answer

    floor

  4. Which Math method always rounds up?

    Show answer

    ceil

  5. In new Date(2026, 0, 15), which month is it?

    Show answer

    January

  6. Which function should generate secure random codes in browsers?

    Show answer

    crypto.getRandomValues

  7. What is Kenya's time zone offset from UTC? Write like +3.

    Show answer

    +3

  8. How many buses of 33 seats are needed for 130 people?

    Show answer

    4

Lesson 4 of 21 in JavaScript · Printable course notes