Date and Time

Creating a date

const now = new Date();
console.log(now);

You can also build a specific one:

const fromString = new Date('2026-09-05');
const fromParts = new Date(2026, 8, 5, 14, 30);
const fromTimestamp = new Date(0);
console.log(fromString, fromParts, fromTimestamp);

The biggest trap: months are counted from zero. January is 0 and December is 11. So new Date(2026, 8, 5) is the fifth of September, not August. Days of the month are normal, starting at 1.

Reading parts of a date

const d = new Date(2026, 8, 5, 14, 30, 45);
console.log(d.getFullYear());
console.log(d.getMonth());
console.log(d.getDate());
console.log(d.getDay());
console.log(d.getHours());
console.log(d.getMinutes());
console.log(d.getSeconds());

Two more traps hide in there:

• getMonth() returns 0 to 11, so add 1 when you display it.
• getDate() is the day of the month, while getDay() is the day of the week where Sunday is 0.

const days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
const d = new Date();
console.log(days[d.getDay()]);

Timestamps

A timestamp is the number of milliseconds since 1 January 1970.

const now = new Date();
console.log(now.getTime());
console.log(Date.now());

Timestamps are plain numbers, which makes them perfect for comparing and sorting.

Comparing dates

const a = new Date('2026-01-01');
const b = new Date('2026-06-01');
console.log(a < b);
console.log(a.getTime() === b.getTime());

Comparison operators work directly, but equality does not, because two Date objects are never the same object. Compare getTime() values instead.

Doing date maths

Subtracting two dates gives milliseconds.

const start = new Date('2026-01-01');
const end = new Date('2026-01-31');
const diffMs = end - start;
const diffDays = diffMs / (1000 * 60 * 60 * 24);
console.log(diffDays);

To add days, use setDate. It handles month and year rollover for you.

const date = new Date('2026-01-30');
date.setDate(date.getDate() + 5);
console.log(date.toDateString());

Adding 5 to 30 January correctly gives 4 February.

Careful: setDate changes the original date object. Copy it first if you need to keep the old value:

const original = new Date('2026-01-30');
const copy = new Date(original);
copy.setDate(copy.getDate() + 5);
console.log(original.toDateString(), copy.toDateString());

Formatting

The built in toString output is not something you want to show users.

const d = new Date(2026, 8, 5);
console.log(d.toString());
console.log(d.toDateString());
console.log(d.toISOString());
console.log(d.toLocaleDateString('en-IN'));
console.log(d.toLocaleDateString('en-US'));

toLocaleDateString accepts options for full control:

const d = new Date(2026, 8, 5);
console.log(d.toLocaleDateString('en-IN', {
  weekday: 'long',
  day: 'numeric',
  month: 'long',
  year: 'numeric'
}));

For times:

const d = new Date();
console.log(d.toLocaleTimeString('en-IN'));
console.log(d.toLocaleString('en-IN'));

Building a format by hand

Sometimes you need an exact shape, like 2026-09-05 for an input field.

function toInputDate(date) {
  const year = date.getFullYear();
  const month = String(date.getMonth() + 1).padStart(2, '0');
  const day = String(date.getDate()).padStart(2, '0');
  return `${year}-${month}-${day}`;
}
console.log(toInputDate(new Date(2026, 8, 5)));

This is where padStart from the strings topic earns its keep.

Parsing strings carefully

console.log(new Date('2026-09-05'));
console.log(new Date('05/09/2026'));
console.log(new Date('not a date'));

Only the ISO format YYYY-MM-DD is reliable across browsers. Slash formats are ambiguous, since 05/09 could be 5 September or 9 May depending on the region. An unparseable string gives Invalid Date.

Check before you trust it:

const d = new Date('not a date');
if (Number.isNaN(d.getTime())) {
  console.log('That date is not valid');
}

A note on the future

The Date API is old and awkward. A modern replacement called Temporal is arriving in browsers and is already available in recent Node.js versions. Learn Date first, since it is everywhere, and reach for Temporal or a small library when you need heavy date work.

Quick recap

• Months are zero based, so add 1 before displaying.
• getDate() is the day of the month, getDay() is the day of the week.
• Date.now() gives a millisecond timestamp, ideal for comparing.
• Subtracting dates gives milliseconds, so divide to get days.
• setDate mutates the original, so copy first.
• Use toLocaleDateString for display and ISO YYYY-MM-DD for parsing.
• Check Number.isNaN(date.getTime()) to detect an invalid date.