JavaScript Date & Time Cheat Sheet

🟢 Create a New Date

new Date() gives current date & time
Pass a date string to create a specific date

const now = new Date();
const specific = new Date('2025-04-05T12:00:00');

📅 Get Date Parts

Extract individual parts: year, month, day, etc.
Remember: getMonth() is 0-based (0=Jan)
getDay() returns weekday (0=Sun)

const date = new Date();
date.getFullYear(); // 2025
date.getMonth();    // 0-11
date.getDate();     // Day of month
date.getDay();      // 0-6 (Sun-Sat)
date.getHours();    // 0-23
date.getMinutes();  // 0-59
date.getSeconds();  // 0-59

📝 Format Date Manually

Concatenate parts yourself
Adjust month by adding 1 (since it’s 0-based)

const date = new Date();
const formatted = `${date.getFullYear()}-${
  date.getMonth() + 1
}-${date.getDate()}`;
console.log(formatted); // e.g., "2025-4-5"

🌐 Format with Intl.DateTimeFormat

Use Intl.DateTimeFormat for localized formatting
Supports options like dateStyle & timeStyle

const formatter = new Intl.DateTimeFormat('en-US', {
  dateStyle: 'full',
  timeStyle: 'short',
});
formatter.format(new Date());
// Example: "Saturday, April 5, 2025 at 12:30 PM"

➕➖ Add or Subtract Days

Safely add/subtract days by modifying a copy

const addDays = (date, days) => {
  const copy = new Date(date);
  copy.setDate(copy.getDate() + days);
  return copy;
};

const today = new Date();
const tomorrow = addDays(today, 1);
console.log(tomorrow);

🔎 Compare Dates

Check if one date is before another
Compare by toDateString() to ignore time

const isBefore = (a, b) => new Date(a) < new Date(b);

const isSameDay = (a, b) =>
  new Date(a).toDateString() === new Date(b).toDateString();

console.log(isBefore('2025-04-01', '2025-05-01')); // true

🌍 Get Timezone Offset

getTimezoneOffset() returns offset in minutes
Negative for UTC+, positive for UTC-

const offsetMinutes = new Date().getTimezoneOffset();
console.log(offsetMinutes); // e.g., -420 for PDT

🔗 Convert to ISO String

toISOString() gives UTC ISO 8601 format
Great for APIs & storage

const iso = new Date().toISOString();
console.log(iso); // e.g., "2025-04-05T19:34:00.000Z"

📥 Parse ISO String

Create a date from an ISO string

const date = new Date("2025-04-05T19:34:00.000Z");
console.log(date);

⏱ Time Since (Relative Time)

Show human-friendly time differences
Calculate seconds, minutes, hours, days ago

const timeSince = date => {
  const seconds = Math.floor((new Date() - new Date(date)) / 1000);

  if (seconds < 60) return `${seconds} sec ago`;

  const minutes = Math.floor(seconds / 60);
  if (minutes < 60) return `${minutes} min ago`;

  const hours = Math.floor(minutes / 60);
  if (hours < 24) return `${hours} hrs ago`;

  const days = Math.floor(hours / 24);
  return `${days} days ago`;
};

console.log(timeSince('2025-04-05T12:00:00'));

📄 Quick Reference

🛠️ Method / Concept	📋 Description	✅ Example
`new Date()`	Create current date/time	`const now = new Date()`
`new Date(string)`	Create from date string	`new Date("2025-04-05T12:00:00")`
`getFullYear()`	Get 4-digit year	`date.getFullYear()` → `2025`
`getMonth()`	Get month (0–11)	`date.getMonth()` → `3` (April)
`getDate()`	Day of the month	`date.getDate()` → `5`
`getDay()`	Weekday (0–6, Sunday = 0)	`date.getDay()` → `6` (Saturday)
`getHours()`	Hours (0–23)	`date.getHours()` → `14`
`getMinutes()`	Minutes (0–59)	`date.getMinutes()` → `45`
`toISOString()`	Convert to ISO string (UTC)	`date.toISOString()` → `"2025-04-05T19:34:00.000Z"`
`getTimezoneOffset()`	Minutes behind/ahead of UTC	`date.getTimezoneOffset()` → `-420` (PDT)
`Intl.DateTimeFormat`	Localized format	`formatter.format(date)`
`setDate()`	Set day of the month	`date.setDate(date.getDate() + 7)`
`toDateString()`	Just the date (no time)	`date.toDateString()` → `"Sat Apr 05 2025"`
`new Date().getTime()`	Timestamp in milliseconds	`Date.now()` → `1722989760000`
`new Date("...") < new Date("...")`	Compare two dates	`isBefore(a, b)`
`timeSince(date)`	Human-friendly "ago" time	`5 min ago`, `2 days ago`