JavaScript Date and Time Cheat Sheet

A beginner-friendly guide to JavaScript date and time. Learn how to create, format, and manipulate dates, work with timestamps, and perform common date calculations.

💡 Key Takeaways

⏰ Creating Dates: new Date() creates a date object from string, timestamp, or components.
⚡ Modify Dates: Use setFullYear(), setMonth(), etc., to change parts of a date.
🌎 Timestamps & UTC: Date.now() gives milliseconds since 1970-01-01; use getUTC*() for UTC times.
🎯 Formatting: toLocaleDateString(), toISOString(), and toTimeString() make dates readable.

🔹 Creating Dates

Create current date/time with new Date()
Use timestamps (ms since Jan 1 1970)
Build custom dates with year, month, day, etc.

const now = new Date(); // current
const fromMs = new Date(1697040000000);
const custom = new Date(2025, 9, 11, 15, 30);

🔢 Accessing Date Components

Use getters like getFullYear(), getMonth(), getDate()
getDay() returns weekday (0 = Sun)
getHours(), getMinutes(), getSeconds() for time

const now = new Date();
console.log(now.getFullYear(), now.getMonth(), now.getDate());
console.log(now.getDay(), now.getHours(), now.getMinutes());

✏️ Modifying Dates

Use setters like setFullYear(), setMonth(), setDate()
Change hours, minutes, or seconds directly
Great for scheduling or adjustments

const date = new Date();
date.setFullYear(2026);
date.setMonth(0); // January
date.setDate(1);
date.setHours(12, 0, 0);

🌎 Timestamps and UTC

Date.now() → current timestamp (ms)
Use getUTC*() for universal time
Convert timestamps back to Date

const ts = Date.now();
const now = new Date(ts);
console.log(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate());

📝 Formatting Dates

toISOString() → standardized output
toLocaleDateString() → user’s region format
Combine parts manually for custom strings

const now = new Date();
console.log(now.toISOString());
console.log(now.toLocaleString());
const custom = `${now.getFullYear()}-${now.getMonth()+1}-${now.getDate()}`;

🔄 Date Math

Add or subtract days with setDate()
Subtract two dates to get difference in ms
Convert milliseconds → days/hours easily

const today = new Date();
today.setDate(today.getDate() + 5);

const d1 = new Date("2025-10-11");
const d2 = new Date("2025-10-01");
const diffDays = (d1 - d2) / (1000 * 60 * 60 * 24);