How Can I Efficiently Calculate the Number of Days Between Two Dates in JavaScript?

How to Compute Days Between Two Dates in JavaScript

Determining the number of days between two dates is a common task in JavaScript. This article presents an efficient solution to this problem.

datediff Function

The datediff function takes two dates as input and returns the number of days between them. Here's a sample implementation:

function datediff(first, second) {
  return Math.round((second - first) / (1000 * 60 * 60 * 24));
}

This function subtracts the milliseconds between the two dates, then divides the result by the number of milliseconds in a day to get the number of days. Rounding to the nearest whole number ensures accuracy, even when handling daylight saving time.

Date Parsing

To parse date strings in a browser-compatible manner, we'll create a simple parseDate function:

function parseDate(str) {
  var mdy = str.split('/');
  return new Date(mdy[2], mdy[0] - 1, mdy[1]);
}

Example Usage

Let's consider the example provided:

<input>

When executing this code, the alert will display the number of days between January 1, 2000, and January 1, 2001, which in this case, is 366 (leap year).

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