Find the day, week, month, or quarter of the year using JavaScript

JavaScript's Date API lacks a lot of methods for working with dates, which is why third-party date libraries are so popular. However, operations such as finding the day, week, month, or quarter of the year that a date corresponds to can be easily implemented without the use of libraries.

Day of year

Finding the day of the year (in the range 1-366) from a Date object is fairly straightforward. We can use the Date constructor and Date.prototype.getFullYear() to get the first day of the year as a Date object.

Then, we can subtract the first day of the year from the given date and divide with the milliseconds in each day to get the result. Finally, we can use Math.floor() to appropriately round the resulting day count to an integer.

const dayOfYear = date =>
  Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 86_400_000);

dayOfYear(new Date('2024-09-28')); // 272

Week of year

Calculating the week of the year also starts by calculating the first day of the year as a Date object. We can then use Date.prototype.setDate(), Date.prototype.getDate() and Date.prototype.getDay() along with the modulo (%) operator to get the first Monday of the year.

Finally, we can subtract the first Monday of the year from the given date and divide with the number of milliseconds in a week. We can use Math.round() to get the zero-indexed week of the year corresponding to the given date. Negative zero (-0) is returned if the given date is before the first Monday of the year.

const weekOfYear = date => {
  const startOfYear = new Date(date.getFullYear(), 0, 1);
  startOfYear.setDate(startOfYear.getDate() + (startOfYear.getDay() % 7));
  return Math.round((date - startOfYear) / 604_800_000);
};

weekOfYear(new Date('2021-06-18')); // 23

Month of year

Finding the month of the year (in the range 1-12) from a Date object is the most straightforward of the bunch. Simply use Date.prototype.getMonth() to get the current month in the range 0-11 and add 1 to map it to the range 1-12.

const monthOfYear = date => date.getMonth() + 1;

monthOfYear(new Date('2024-09-28')); // 9

Quarter of year

Finding the quarter of the year (in the range 1-4) from a Date is quite simple, too. After retrieving the current month, we can use Math.ceil() and divide the month by 3 to get the current quarter.

const quarterOfYear = date => Math.ceil((date.getMonth() + 1) / 3);

quarterOfYear(new Date('2024-09-28')); // 3