JavaScript Date Time Interview Preparation Frontend Development

JavaScript Date & Time

Master JavaScript dates — Unix epoch, UTC vs local time, creating and parsing Date objects, getters and setters, arithmetic, common pitfalls, and production best practices with ISO 8601.

Jul 3, 202612 min read

Introduction

Dates look simple until you ship them. A user in Dhaka, a server in Virginia, and a database storing "local time" without a zone will eventually produce bugs that only appear on the last Sunday in October.

JavaScript stores every Date internally as milliseconds since the Unix epoch (1 January 1970, 00:00:00 UTC). What you see in the console is almost always converted to the local time zone of the runtime — browser, Node.js, or test runner.

This guide covers:

Core concepts — epoch, timestamps, UTC vs local
Key terminology — UTC, GMT, time zones, and milliseconds
Creating dates — constructors, Date.now(), and ISO 8601
Reading & writing — getters, setters, and UTC equivalents
Parsing & validation — safe parsing and invalid-date checks
Arithmetic — adding time and measuring duration
Common pitfalls — month index, DST, and parsing quirks
Best practices — storage, display, and when to reach for a library

Each section includes runnable examples for the browser console and Node.js.

Course Reference

Quick index

#	Section
1	Core concepts
2	Key terminology
3	Creating Date objects
4	Getters & setters
5	Parsing & validation
6	Date arithmetic
7	Common pitfalls
8	Best practices

1. Core concepts

Idea	Detail
Unix epoch	1 January 1970, 00:00:00 UTC — the zero point for timestamps
Internal storage	A `Date` holds milliseconds since epoch as a Number (UTC-based)
Display	`toString()`, `getHours()`, etc. convert to the local time zone
Time zone complexity	Offsets, DST, and political changes make manual math error-prone

javascript

const now = new Date()
console.log(now.getTime()) // 1730000000000 — UTC milliseconds (example)
console.log(now.toString()) // local representation, e.g. "Fri Jul 03 2026 ..."
console.log(now.toISOString()) // always UTC: "2026-07-03T09:52:00.000Z"

Think of a Date as a UTC instant on a timeline. Local getters project that instant onto your machine's time zone; UTC getters read the instant directly.

javascript

const d = new Date()
console.log(d.toISOString()) // UTC
console.log(d.getTimezoneOffset()) // e.g. -360 for UTC+6 (Dhaka)

Back to index

2. Key terminology

Epoch time

The universal starting point for computer clocks: 1 January 1970, 00:00:00 UTC. Every timestamp counts forward from that moment.

Timestamp

A numeric count of time units since the epoch. JavaScript uses milliseconds; many Unix tools use seconds.

javascript

const ms = Date.now()
const seconds = Math.floor(ms / 1000)
 
console.log(ms) // 1730000000000
console.log(seconds) // 1730000000

UTC and GMT

UTC (Coordinated Universal Time) is the modern standard. GMT is often used interchangeably in casual speech. Neither observes daylight saving — they are stable baselines for storage and APIs.

Time zones

A region's offset from UTC, e.g. UTC+6 for Bangladesh Standard Time. Offsets can change with DST (not applicable in Bangladesh, but relevant for US/EU users).

javascript

// Intl gives correct zone-aware formatting without manual offset math
const formatter = new Intl.DateTimeFormat('en-BD', {
  timeZone: 'Asia/Dhaka',
  dateStyle: 'full',
  timeStyle: 'long',
})
 
console.log(formatter.format(new Date()))
// "Friday, 3 July 2026 at 3:52:00 pm GMT+6"

Back to index

3. Creating Date objects

Current date and time

javascript

const now = new Date() // current instant
const timestamp = Date.now() // same instant as number — preferred for performance

From a timestamp (milliseconds)

javascript

const d = new Date(1_700_000_000_000)
console.log(d.toISOString()) // "2023-11-14T22:13:20.000Z"

From an ISO 8601 string (recommended)

javascript

const d = new Date('2026-07-03T09:30:00.000Z')
console.log(d.getUTCFullYear()) // 2026
console.log(d.getUTCMonth()) // 6 (July — see pitfalls)
console.log(d.getUTCDate()) // 3

ISO format: YYYY-MM-DDTHH:mm:ss.sssZ — the trailing Z means UTC.

From components (local time)

javascript

// year, monthIndex, day, hours, minutes, seconds, ms
const birthday = new Date(1995, 6, 15) // 15 July 1995 — local midnight
console.log(birthday.getFullYear()) // 1995
console.log(birthday.getMonth()) // 6
console.log(birthday.getDate()) // 15

javascript

const local = new Date(2026, 0, 1) // 1 Jan 2026, 00:00 local
const utcMs = Date.UTC(2026, 0, 1) // 1 Jan 2026, 00:00 UTC as milliseconds
const utc = new Date(utcMs)
 
console.log(local.toString())
console.log(utc.toISOString())

Static helpers

javascript

Date.now() // current timestamp (ms)
Date.parse('2026-07-03') // parse to timestamp — implementation-dependent for non-ISO strings
Date.UTC(2026, 6, 3) // UTC ms for 3 July 2026 00:00:00 UTC

Back to index

4. Getters & setters

Local getters

javascript

const d = new Date('2026-07-03T14:30:00.000Z')
 
console.log(d.getFullYear()) // local year
console.log(d.getMonth()) // 0–11
console.log(d.getDate()) // day of month 1–31
console.log(d.getDay()) // day of week 0–6 (Sun–Sat)
console.log(d.getHours())
console.log(d.getMinutes())
console.log(d.getSeconds())
console.log(d.getMilliseconds())

UTC getters

Use UTC methods when the source data is UTC (APIs, databases) and you want stable values regardless of the user's zone:

javascript

const d = new Date('2026-07-03T14:30:00.000Z')
 
console.log(d.getUTCFullYear()) // 2026
console.log(d.getUTCMonth()) // 6
console.log(d.getUTCDate()) // 3
console.log(d.getUTCHours()) // 14
console.log(d.getUTCMinutes()) // 30

Setters (mutate in place)

javascript

const d = new Date()
d.setFullYear(2027)
d.setMonth(11) // December
d.setDate(25)
d.setHours(0, 0, 0, 0) // midnight local
 
console.log(d.toString())

UTC setters mirror local ones: setUTCFullYear, setUTCMonth, setUTCDate, setUTCHours, etc.

Tip

Date objects are mutable. Calling setMonth changes the same object. Clone before mutating if you need the original:

javascript

const original = new Date()
const copy = new Date(original.getTime())
copy.setDate(copy.getDate() + 7)

Useful output methods

javascript

const d = new Date('2026-07-03T09:00:00.000Z')
 
d.toISOString() // "2026-07-03T09:00:00.000Z" — best for APIs/DB
d.toJSON() // same as toISOString()
d.toDateString() // "Fri Jul 03 2026"
d.toTimeString() // local time with zone name
d.toLocaleDateString('en-BD') // locale-aware display
d.toLocaleString('en-BD', { timeZone: 'Asia/Dhaka' })

Back to index

5. Parsing & validation

Prefer ISO strings

javascript

const good = new Date('2026-07-03T10:00:00Z')
const alsoGood = new Date('2026-07-03') // parsed as UTC midnight in modern engines

Avoid ambiguous formats

javascript

// ❌ Ambiguous — US vs EU date order
new Date('03/07/2026') // could be 3 Jul or 7 Mar depending on engine/locale
 
// ✅ Unambiguous
new Date('2026-07-03')

Detect invalid dates

javascript

function isValidDate(value) {
  const d = value instanceof Date ? value : new Date(value)
  return !Number.isNaN(d.getTime())
}
 
console.log(isValidDate('2026-07-03')) // true
console.log(isValidDate('not a date')) // false
console.log(isValidDate(new Date('invalid'))) // false

Safe parsing helper

javascript

function parseISODate(isoString) {
  const d = new Date(isoString)
  if (Number.isNaN(d.getTime())) {
    throw new RangeError(`Invalid ISO date: ${isoString}`)
  }
  return d
}
 
const published = parseISODate('2026-07-03T09:00:00.000Z')

Back to index

6. Date arithmetic

Work in milliseconds for addition and subtraction — avoid manually adding days to getDate() across month boundaries.

Constants

javascript

const SECOND = 1000
const MINUTE = 60 * SECOND
const HOUR = 60 * MINUTE
const DAY = 24 * HOUR

Add duration

javascript

function addDays(date, days) {
  return new Date(date.getTime() + days * DAY)
}
 
const today = new Date('2026-07-03T12:00:00Z')
const nextWeek = addDays(today, 7)
console.log(nextWeek.toISOString()) // 2026-07-10T12:00:00.000Z

Difference between two dates

javascript

function diffInDays(a, b) {
  const ms = Math.abs(b.getTime() - a.getTime())
  return Math.floor(ms / DAY)
}
 
const start = new Date('2026-01-01')
const end = new Date('2026-07-03')
console.log(diffInDays(start, end)) // 183

Human-readable duration

javascript

function formatDuration(ms) {
  const totalSeconds = Math.floor(ms / 1000)
  const hours = Math.floor(totalSeconds / 3600)
  const minutes = Math.floor((totalSeconds % 3600) / 60)
  const seconds = totalSeconds % 60
  return `${hours}h ${minutes}m ${seconds}s`
}
 
const started = new Date('2026-07-03T08:00:00Z')
const finished = new Date('2026-07-03T10:45:30Z')
console.log(formatDuration(finished - started)) // "2h 45m 30s"

Back to index

7. Common pitfalls

Month indexing (0–11)

January is 0, December is 11. This trips up almost every developer at least once.

javascript

const wrong = new Date(2026, 7, 1) // 1 August 2026 — not July!
const right = new Date(2026, 6, 1) // 1 July 2026
 
console.log(wrong.getMonth()) // 7
console.log(right.getMonth()) // 6

`setMonth` overflow

javascript

const d = new Date(2026, 0, 31) // 31 January
d.setMonth(1) // "February 31" rolls to March
console.log(d.toDateString()) // "Tue Mar 03 2026" in most engines

Always validate day-of-month when incrementing months, or use a library.

Daylight saving time (DST)

In zones with DST, some local times do not exist (spring forward) or occur twice (fall back). Manual offset math breaks.

javascript

// ❌ Fragile — assumes every day has 24 hours in local time
const tomorrow = new Date()
tomorrow.setDate(tomorrow.getDate() + 1)
 
// ✅ Prefer UTC ms arithmetic for fixed durations
const in24Hours = new Date(Date.now() + 24 * 60 * 60 * 1000)

Comparing dates

javascript

const a = new Date('2026-07-03')
const b = new Date('2026-07-03')
 
console.log(a === b) // false — different object references
console.log(a.getTime() === b.getTime()) // true — same instant
console.log(a <= b) // true — coercion to number works

Back to index

8. Best practices

Store UTC, display local

Layer	Recommendation
Database	`TIMESTAMPTZ` or ISO 8601 UTC strings
API	Always serialize with `toISOString()`
UI	Format with `Intl` or locale helpers for the user

javascript

// API response handler
function serializeForApi(date) {
  return date.toISOString()
}
 
// UI display
function displayLocal(isoUtc) {
  return new Intl.DateTimeFormat(undefined, {
    dateStyle: 'medium',
    timeStyle: 'short',
  }).format(new Date(isoUtc))
}
 
displayLocal('2026-07-03T09:00:00.000Z')
// e.g. "3 Jul 2026, 3:00 pm" in Asia/Dhaka

Use ISO 8601 for wire format

javascript

const payload = {
  title: 'New post',
  publishedAt: new Date().toISOString(),
}
JSON.stringify(payload)
// {"title":"New post","publishedAt":"2026-07-03T09:52:00.000Z"}

When to use a library

Task	Native `Date`	Library (date-fns / Luxon)
Current timestamp	✅	✅
ISO parse/format	✅	✅
Add days in UTC	✅	✅
Named time zones	⚠️ `Intl`	✅
Recurring rules (RRULE)	❌	✅
Business days / holidays	❌	✅

javascript

// date-fns example
import { addDays, format, parseISO, isValid } from 'date-fns'
 
const base = parseISO('2026-07-03')
const next = addDays(base, 7)
console.log(format(next, 'yyyy-MM-dd')) // "2026-07-10"
console.log(isValid(parseISO('bad'))) // false

Back to index

Summary

JavaScript dates are UTC instants dressed in local clothing. Create them with ISO strings or timestamps, read them with the right getter (local vs UTC), mutate carefully, and do math in milliseconds. The month index, DST, and ambiguous parsers cause most real-world bugs — store UTC, show local, and escalate to Intl or date-fns when time zones get serious.

Related reads: JavaScript Strings for formatting patterns, and JavaScript Code Snippets for a reusable date-formatting utility.

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JavaScript Date Time Interview Preparation Frontend Development

JavaScript Date & Time

Master JavaScript dates — Unix epoch, UTC vs local time, creating and parsing Date objects, getters and setters, arithmetic, common pitfalls, and production best practices with ISO 8601.

Jul 3, 202612 min read

Introduction

This guide covers:

Core concepts — epoch, timestamps, UTC vs local
Key terminology — UTC, GMT, time zones, and milliseconds
Creating dates — constructors, Date.now(), and ISO 8601
Reading & writing — getters, setters, and UTC equivalents
Parsing & validation — safe parsing and invalid-date checks
Arithmetic — adding time and measuring duration
Common pitfalls — month index, DST, and parsing quirks
Best practices — storage, display, and when to reach for a library

Each section includes runnable examples for the browser console and Node.js.

Course Reference

Quick index

#	Section
1	Core concepts
2	Key terminology
3	Creating Date objects
4	Getters & setters
5	Parsing & validation
6	Date arithmetic
7	Common pitfalls
8	Best practices

1. Core concepts

Idea	Detail
Unix epoch	1 January 1970, 00:00:00 UTC — the zero point for timestamps
Internal storage	A `Date` holds milliseconds since epoch as a Number (UTC-based)
Display	`toString()`, `getHours()`, etc. convert to the local time zone
Time zone complexity	Offsets, DST, and political changes make manual math error-prone

javascript

const now = new Date()
console.log(now.getTime()) // 1730000000000 — UTC milliseconds (example)
console.log(now.toString()) // local representation, e.g. "Fri Jul 03 2026 ..."
console.log(now.toISOString()) // always UTC: "2026-07-03T09:52:00.000Z"

Think of a Date as a UTC instant on a timeline. Local getters project that instant onto your machine's time zone; UTC getters read the instant directly.

javascript

const d = new Date()
console.log(d.toISOString()) // UTC
console.log(d.getTimezoneOffset()) // e.g. -360 for UTC+6 (Dhaka)

Back to index

2. Key terminology

Epoch time

The universal starting point for computer clocks: 1 January 1970, 00:00:00 UTC. Every timestamp counts forward from that moment.

Timestamp

A numeric count of time units since the epoch. JavaScript uses milliseconds; many Unix tools use seconds.

javascript

const ms = Date.now()
const seconds = Math.floor(ms / 1000)
 
console.log(ms) // 1730000000000
console.log(seconds) // 1730000000

UTC and GMT

Time zones

A region's offset from UTC, e.g. UTC+6 for Bangladesh Standard Time. Offsets can change with DST (not applicable in Bangladesh, but relevant for US/EU users).

javascript

// Intl gives correct zone-aware formatting without manual offset math
const formatter = new Intl.DateTimeFormat('en-BD', {
  timeZone: 'Asia/Dhaka',
  dateStyle: 'full',
  timeStyle: 'long',
})
 
console.log(formatter.format(new Date()))
// "Friday, 3 July 2026 at 3:52:00 pm GMT+6"

Back to index

3. Creating Date objects

Current date and time

javascript

const now = new Date() // current instant
const timestamp = Date.now() // same instant as number — preferred for performance

From a timestamp (milliseconds)

javascript

const d = new Date(1_700_000_000_000)
console.log(d.toISOString()) // "2023-11-14T22:13:20.000Z"

From an ISO 8601 string (recommended)

javascript

const d = new Date('2026-07-03T09:30:00.000Z')
console.log(d.getUTCFullYear()) // 2026
console.log(d.getUTCMonth()) // 6 (July — see pitfalls)
console.log(d.getUTCDate()) // 3

ISO format: YYYY-MM-DDTHH:mm:ss.sssZ — the trailing Z means UTC.

From components (local time)

javascript

// year, monthIndex, day, hours, minutes, seconds, ms
const birthday = new Date(1995, 6, 15) // 15 July 1995 — local midnight
console.log(birthday.getFullYear()) // 1995
console.log(birthday.getMonth()) // 6
console.log(birthday.getDate()) // 15

javascript

const local = new Date(2026, 0, 1) // 1 Jan 2026, 00:00 local
const utcMs = Date.UTC(2026, 0, 1) // 1 Jan 2026, 00:00 UTC as milliseconds
const utc = new Date(utcMs)
 
console.log(local.toString())
console.log(utc.toISOString())

Static helpers

javascript

Date.now() // current timestamp (ms)
Date.parse('2026-07-03') // parse to timestamp — implementation-dependent for non-ISO strings
Date.UTC(2026, 6, 3) // UTC ms for 3 July 2026 00:00:00 UTC

Back to index

4. Getters & setters

Local getters

javascript

const d = new Date('2026-07-03T14:30:00.000Z')
 
console.log(d.getFullYear()) // local year
console.log(d.getMonth()) // 0–11
console.log(d.getDate()) // day of month 1–31
console.log(d.getDay()) // day of week 0–6 (Sun–Sat)
console.log(d.getHours())
console.log(d.getMinutes())
console.log(d.getSeconds())
console.log(d.getMilliseconds())

UTC getters

Use UTC methods when the source data is UTC (APIs, databases) and you want stable values regardless of the user's zone:

javascript

const d = new Date('2026-07-03T14:30:00.000Z')
 
console.log(d.getUTCFullYear()) // 2026
console.log(d.getUTCMonth()) // 6
console.log(d.getUTCDate()) // 3
console.log(d.getUTCHours()) // 14
console.log(d.getUTCMinutes()) // 30

Setters (mutate in place)

javascript

const d = new Date()
d.setFullYear(2027)
d.setMonth(11) // December
d.setDate(25)
d.setHours(0, 0, 0, 0) // midnight local
 
console.log(d.toString())

UTC setters mirror local ones: setUTCFullYear, setUTCMonth, setUTCDate, setUTCHours, etc.

Tip

Date objects are mutable. Calling setMonth changes the same object. Clone before mutating if you need the original:

javascript

const original = new Date()
const copy = new Date(original.getTime())
copy.setDate(copy.getDate() + 7)

Useful output methods

javascript

const d = new Date('2026-07-03T09:00:00.000Z')
 
d.toISOString() // "2026-07-03T09:00:00.000Z" — best for APIs/DB
d.toJSON() // same as toISOString()
d.toDateString() // "Fri Jul 03 2026"
d.toTimeString() // local time with zone name
d.toLocaleDateString('en-BD') // locale-aware display
d.toLocaleString('en-BD', { timeZone: 'Asia/Dhaka' })

Back to index

5. Parsing & validation

Prefer ISO strings

javascript

const good = new Date('2026-07-03T10:00:00Z')
const alsoGood = new Date('2026-07-03') // parsed as UTC midnight in modern engines

Avoid ambiguous formats

javascript

// ❌ Ambiguous — US vs EU date order
new Date('03/07/2026') // could be 3 Jul or 7 Mar depending on engine/locale
 
// ✅ Unambiguous
new Date('2026-07-03')

Detect invalid dates

javascript

function isValidDate(value) {
  const d = value instanceof Date ? value : new Date(value)
  return !Number.isNaN(d.getTime())
}
 
console.log(isValidDate('2026-07-03')) // true
console.log(isValidDate('not a date')) // false
console.log(isValidDate(new Date('invalid'))) // false

Safe parsing helper

javascript

function parseISODate(isoString) {
  const d = new Date(isoString)
  if (Number.isNaN(d.getTime())) {
    throw new RangeError(`Invalid ISO date: ${isoString}`)
  }
  return d
}
 
const published = parseISODate('2026-07-03T09:00:00.000Z')

Back to index

6. Date arithmetic

Work in milliseconds for addition and subtraction — avoid manually adding days to getDate() across month boundaries.

Constants

javascript

const SECOND = 1000
const MINUTE = 60 * SECOND
const HOUR = 60 * MINUTE
const DAY = 24 * HOUR

Add duration

javascript

function addDays(date, days) {
  return new Date(date.getTime() + days * DAY)
}
 
const today = new Date('2026-07-03T12:00:00Z')
const nextWeek = addDays(today, 7)
console.log(nextWeek.toISOString()) // 2026-07-10T12:00:00.000Z

Difference between two dates

javascript

function diffInDays(a, b) {
  const ms = Math.abs(b.getTime() - a.getTime())
  return Math.floor(ms / DAY)
}
 
const start = new Date('2026-01-01')
const end = new Date('2026-07-03')
console.log(diffInDays(start, end)) // 183

Human-readable duration

javascript

function formatDuration(ms) {
  const totalSeconds = Math.floor(ms / 1000)
  const hours = Math.floor(totalSeconds / 3600)
  const minutes = Math.floor((totalSeconds % 3600) / 60)
  const seconds = totalSeconds % 60
  return `${hours}h ${minutes}m ${seconds}s`
}
 
const started = new Date('2026-07-03T08:00:00Z')
const finished = new Date('2026-07-03T10:45:30Z')
console.log(formatDuration(finished - started)) // "2h 45m 30s"

Back to index

7. Common pitfalls

Month indexing (0–11)

January is 0, December is 11. This trips up almost every developer at least once.

javascript

const wrong = new Date(2026, 7, 1) // 1 August 2026 — not July!
const right = new Date(2026, 6, 1) // 1 July 2026
 
console.log(wrong.getMonth()) // 7
console.log(right.getMonth()) // 6

`setMonth` overflow

javascript

const d = new Date(2026, 0, 31) // 31 January
d.setMonth(1) // "February 31" rolls to March
console.log(d.toDateString()) // "Tue Mar 03 2026" in most engines

Always validate day-of-month when incrementing months, or use a library.

Daylight saving time (DST)

In zones with DST, some local times do not exist (spring forward) or occur twice (fall back). Manual offset math breaks.

javascript

// ❌ Fragile — assumes every day has 24 hours in local time
const tomorrow = new Date()
tomorrow.setDate(tomorrow.getDate() + 1)
 
// ✅ Prefer UTC ms arithmetic for fixed durations
const in24Hours = new Date(Date.now() + 24 * 60 * 60 * 1000)

Comparing dates

javascript

const a = new Date('2026-07-03')
const b = new Date('2026-07-03')
 
console.log(a === b) // false — different object references
console.log(a.getTime() === b.getTime()) // true — same instant
console.log(a <= b) // true — coercion to number works

Back to index

8. Best practices

Store UTC, display local

Layer	Recommendation
Database	`TIMESTAMPTZ` or ISO 8601 UTC strings
API	Always serialize with `toISOString()`
UI	Format with `Intl` or locale helpers for the user

javascript

// API response handler
function serializeForApi(date) {
  return date.toISOString()
}
 
// UI display
function displayLocal(isoUtc) {
  return new Intl.DateTimeFormat(undefined, {
    dateStyle: 'medium',
    timeStyle: 'short',
  }).format(new Date(isoUtc))
}
 
displayLocal('2026-07-03T09:00:00.000Z')
// e.g. "3 Jul 2026, 3:00 pm" in Asia/Dhaka

Use ISO 8601 for wire format

javascript

const payload = {
  title: 'New post',
  publishedAt: new Date().toISOString(),
}
JSON.stringify(payload)
// {"title":"New post","publishedAt":"2026-07-03T09:52:00.000Z"}

When to use a library

Task	Native `Date`	Library (date-fns / Luxon)
Current timestamp	✅	✅
ISO parse/format	✅	✅
Add days in UTC	✅	✅
Named time zones	⚠️ `Intl`	✅
Recurring rules (RRULE)	❌	✅
Business days / holidays	❌	✅

javascript

// date-fns example
import { addDays, format, parseISO, isValid } from 'date-fns'
 
const base = parseISO('2026-07-03')
const next = addDays(base, 7)
console.log(format(next, 'yyyy-MM-dd')) // "2026-07-10"
console.log(isValid(parseISO('bad'))) // false

Back to index

Summary

Related reads: JavaScript Strings for formatting patterns, and JavaScript Code Snippets for a reusable date-formatting utility.

Introduction

Course Reference

Quick index

1. Core concepts

2. Key terminology

Epoch time

Timestamp

UTC and GMT

Time zones

3. Creating Date objects

Current date and time

From a timestamp (milliseconds)

From an ISO 8601 string (recommended)

From components (local time)

Static helpers

4. Getters & setters

Local getters

UTC getters

Setters (mutate in place)

Useful output methods

5. Parsing & validation

Prefer ISO strings

Avoid ambiguous formats

Detect invalid dates

Safe parsing helper

6. Date arithmetic

Constants

Add duration

Difference between two dates

Human-readable duration

7. Common pitfalls

Month indexing (0–11)

setMonth overflow

Daylight saving time (DST)

Comparing dates

8. Best practices

Store UTC, display local

Use ISO 8601 for wire format

When to use a library

Summary

Subscribe to my newsletter

Introduction

Course Reference

Quick index

1. Core concepts

2. Key terminology

Epoch time

Timestamp

UTC and GMT

Time zones

3. Creating Date objects

Current date and time

From a timestamp (milliseconds)

From an ISO 8601 string (recommended)

From components (local time)

Static helpers

4. Getters & setters

Local getters

UTC getters

Setters (mutate in place)

Useful output methods

5. Parsing & validation

Prefer ISO strings

Avoid ambiguous formats

Detect invalid dates

Safe parsing helper

6. Date arithmetic

Constants

Add duration

Difference between two dates

Human-readable duration

7. Common pitfalls

Month indexing (0–11)

setMonth overflow

Daylight saving time (DST)

Comparing dates

8. Best practices

Store UTC, display local

Use ISO 8601 for wire format

When to use a library

`setMonth` overflow

`setMonth` overflow