JavaScript — Dates

Creating dates

new Date();                       // right now
new Date("2026-08-23");           // from ISO string
new Date(2026, 7, 23);            // year, MONTH?, day — see trap below
new Date(1790000000000);          // milliseconds since epoch (Jan 1 1970)
Date.now();                       // current ms timestamp — cheap & comparable

The month trap

Months are zero-indexed:

new Date(2026, 0, 1);   // Jan 1   (0 = January!)
new Date(2026, 11, 31); // Dec 31  (11 = December!)

Days are 1-based; months are 0-based. Everyone gets bitten once.

Getting parts

const d = new Date();

d.getFullYear();   // 2026
d.getMonth();      // 7        → August (0-based!)
d.getDate();       // 23       day of month
d.getDay();        // 0=Sun…6=Sat
d.getHours();      // 19
d.getMinutes();
d.getSeconds();
d.getTime();       // ms timestamp

Formatting

Built-in locale strings

d.toLocaleDateString();         // "23/08/2026" (per user locale)
d.toLocaleDateString("en-US", { weekday: "long", month: "long", day: "numeric" });
// "Sunday, August 23"
d.toLocaleTimeString("en-GB");  // "19:45:12"
d.toISOString();                // "2026-08-23T19:45:12.999Z" — machine standard

toISOString is what you send to servers/databases; toLocale* is what humans read.

Manual template

const pad = n => String(n).padStart(2, "0");
`${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}`;  // 2026-08-23

Math with dates

Subtract two Dates → milliseconds between them:

const start = new Date("2026-01-01");
const end = new Date("2026-08-23");

const days = Math.round((end - start) / 86400000);   // ~234

Constants: 1000 ms/s · 86400000 ms/day.

Add days safely by mutation:

const later = new Date(start);
later.setDate(later.getDate() + 30);   // auto-rolls across months/years ✓

Parsing pitfalls

new Date("08/23/2026")    // US-format dependent — avoid
new Date("2026-08-23")    // ISO date-only → parsed as UTC midnight ⚠️
new Date("2026-08-23T10:00")  // local time

For anything serious, store ISO strings or timestamps; parse with intent.

Modern alternative

Intl.DateTimeFormat and the newer Temporal proposal handle calendars/timezones rigorously; libraries (date-fns/dayjs) fill production gaps. For tutorials, native Date suffices.

Mini Practice

1. Print today's weekday name via toLocaleDateString options
2. Days-until-New-Year calculator using timestamps
3. Build formatYYYYMMDD(d) with padStart
4. Add one month to Jan 31 — observe rollover behavior; explain it
5. Convert an ISO string to a readable local string both ways

Next: math → (already covered in numbers)