JavaScript: JavaScript Dates

Last updated: 2026-10-13

JavaScript uses the Date object to work with dates and times. Think of it as a digital watch — it shows the current time, lets you set any moment, and can calculate the difference between two times.

Related Date Methods：getFullYear() toISOString() getMonth() getHours() Date.now() getDate() getDay() Date set

1. Creating and Manipulating Date Objects

(1) Creating Date Objects

Four common ways to create a Date:

<script>
console.log(new Date());                      // Current date and time
console.log(new Date("2025-06-19"));          // Date string
console.log(new Date(2025, 5, 19, 10, 30, 0)); // Year, month, day, hours, minutes, seconds
console.log(new Date(1718764800000));         // Timestamp (milliseconds)
</script>

Big gotcha: Months are zero-indexed! 0 = January, 11 = December. In new Date(2025, 5, 19), the 5 means June, not May. This is arguably the most criticized design decision in the Date object — period.

(2) Getting Date and Time

getDay() returns the day of the week, not the day of the month — use getDate() for that. Beginners often mix these up.

(3) Setting Date and Time

There's a matching set of set methods: setFullYear(), setMonth(), setDate(), setHours(), setMinutes(), setSeconds(). Values automatically roll over when set:

<script>
const d = new Date(2025, 5, 19);
d.setDate(32); // Automatically becomes July 2
console.log(d.toLocaleDateString());
</script>

(4) Formatting Dates

These methods automatically format based on the browser's locale.

(5) Timestamps and Calculating Time Differences

A timestamp is the number of milliseconds elapsed since January 1, 1970 00:00:00 UTC.

<script>
console.log(Date.now());           // Get current timestamp
console.log(new Date().getTime()); // Same as above
</script>

The core approach to calculating time differences: subtract two timestamps to get the millisecond difference, then convert:

<script>
const start = Date.now();
for (let i = 0; i < 1000000; i++) {} // Simulate some operation
const end = Date.now();
const diff = end - start;        // Milliseconds
const diffSeconds = diff / 1000; // Seconds
console.log("Time elapsed: " + diff + "ms (" + diffSeconds + " seconds)");
</script>

▶ Example: Displaying Current Date and Time

▶ Example: Date Info Details

▶ Example: Countdown Timer

▶ Example: Date Difference Calculator

Related Date Methods：getFullYear() toISOString() getMonth() getHours() Date.now() getDate() getDay() Date set

❓ FAQ

📖 Summary

• Create Date objects with new Date(), new Date(2024, 0, 1), or new Date("2024-01-01")
• Get methods: getFullYear/getMonth/getDate/getDay/getHours/getMinutes/getSeconds
• Set methods modify the Date object in place
• Format dates with toLocaleDateString(), toISOString(), or manual concatenation
• Timestamps are milliseconds; use Date.now() for current timestamp, add/subtract for date math

📝 Exercises

1. Write a function formatDate(date) that returns a string in the format "YYYY-MM-DD (Weekday)". Make sure months and days are zero-padded.
2. Write a function getAge(birthday) that takes a date string (e.g., "2000-03-15") and returns the current age in full years.
3. Build a "time ago" feature: given a past date, display "X years, X months, X days ago" or "X days ago" or "X hours ago" — automatically choosing the appropriate format based on the time difference.

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