How to compare two dates with JavaScript

Matthew C.

—January 30, 2023

The Problem

You want to compare two dates with JavaScript. How can you do this?

The Solution

To compare two dates, first make a Date object for each date. The Date object is used to represent a single moment in time and has methods for formatting dates and doing time zone conversions.

Next, you can compare the two Date objects using comparison operators (>, <, =, and >=):

const date1 = new Date("December 15, 2022");
const date2 = new Date("December 16, 2022");

if (date1 < date2) {
  console.log("date1 is earlier than date2");
} else if (date1 > date2) {
  console.log("date1 is later than date2");
} else {
  console.log("date1 and date2 are the same");
}
// date1 is earlier than date2

Comparing two Date objects does not work when you use equality operators (==, !=, ===, or !==):

const date1 = new Date("December 15, 2022");
const date2 = new Date("December 15, 2022");
if (date1 === date2) {
  console.log("date1 and date2 are the same");
} else {
  console.log("date1 and date2 are not the same");
}
// date1 and date2 are not the same

Equality operators don’t work when comparing two Date objects, because JavaScript objects are compared by reference, not value. Two Date variables are not considered to be the same unless they point to the same Date object in memory.

When comparing Date objects, it’s best to first convert the Date to a timestamp number or a date string to avoid any issues with equality operators. If the Date constructor is called without the new operator, it ignores all arguments and returns a string representing the current time. It’s the same as calling new Date().toString().

Use the toString() method to return a string representation of the Date object showing the date and time in the user’s time zone. The toString() method is a combination of toDateString() and toTimeString():

console.log(new Date().toString()); // Mon Dec 19 2022 09:51:24 GMT+0200 (South Africa Standard Time)

To get a numerical representation of the Date, you can call the getTime() or valueOf method. Both methods return the number of milliseconds since January 1, 1970, 00:00:00 UTC.

You can also use other Date instance methods to convert Date objects into formatted strings or numbers for comparing. For example, you can get the year, month, week, day, hour, minutes, or seconds of a Date:

const date = new Date("December 22, 2022, 13:42:33");
const [year, month, week, day, hour, minutes, seconds] = [
  date.getFullYear(),
  date.getMonth(),
  date.getDay(),
  date.getDate(),
  date.getHours(),
  date.getMinutes(),
  date.getSeconds(),
];
console.log(year, month, week, day, hour, minutes, seconds); // 2022 11 4 22 13 42 33

The getMonth() method returns the index of the month, which starts at 0. The getDay() method returns the index of the week, which starts at 0 on Sunday.

For more complex comparisons, you can use a library such as date-fns, which has multiple utility functions to help make comparisons easier.

Further Reading

If you’re looking to get a deeper understanding of how JavaScript application monitoring works, take a look at the following articles:

• Understanding the Performance Impact of Generated JavaScript
• Making your JavaScript projects less noisy
• 3 Ways to Improve JavaScript Applications
• JavaScript Debugging Hub
• 11 Habits of Highly Effective Developers