Determining the first and last days of a quarter is a common task for accounting applications and reporting. For web applications, I have been using Day.js (https://day.js.org/) for a long time instead of the well-known moment. There is an extension for Day.js that allows you to determine the first and last days of a quarter.

The extension is called quarterOfYear.

Here is an example of how to use it:

import dayjs from "dayjs";
import quarterOfYear from "dayjs/plugin/quarterOfYear";

dayjs.extend(quarterOfYear);

// First day of current quarter
const firstDayOfQuarter = dayjs().startOf("quarter");

// Last day of current quarter
const lastDayOfQuarter = dayjs().endOf("quarter");

console.log(firstDayOfQuarter.format("YYYY-MM-DD"));
console.log(lastDayOfQuarter.format("YYYY-MM-DD"));

If you run this script today, December 10, 2025, the result will be as follows:

2025-10-01
2025-12-31