How to calculate date difference in JavaScript

Working with dates is hard, but often necessary, which is the reason why there are so many date-related libraries out there. However, some tasks, such as calculating the difference between two dates, can be easily accomplished with vanilla JavaScript.

Date difference in seconds

Calculating the date difference in seconds is as simple as subtracting the two Date objects and dividing by the number of milliseconds in a second (1000).

const dateDifferenceInSeconds = (dateInitial, dateFinal) =>
  (dateFinal - dateInitial) / 1_000;

dateDifferenceInSeconds(
  new Date('2020-12-24 00:00:15'),
  new Date('2020-12-24 00:00:17')
); // 2

Date difference in minutes

Similarly, to calculate the date difference in minutes, we only need to change the divisor to the number of milliseconds in a minute (1000 * 60).

const dateDifferenceInMinutes = (dateInitial, dateFinal) =>
  (dateFinal - dateInitial) / 60_000;

dateDifferenceInMinutes(
  new Date('2021-04-24 01:00:15'),
  new Date('2021-04-24 02:00:15')
); // 60

Date difference in hours

To calculate the date difference in hours, we do the same thing, but with the number of milliseconds in an hour (1000 * 60 * 60).

const dateDifferenceInHours = (dateInitial, dateFinal) =>
  (dateFinal - dateInitial) / 3_600_000;

dateDifferenceInHours(
  new Date('2021-04-24 10:25:00'),
  new Date('2021-04-25 10:25:00')
); // 24

Date difference in days

Subtraction can take us further still, calculating the date difference in days. Without taking timezones into account, we can simply divide the difference in milliseconds by the number of milliseconds in a day (1000 * 60 * 60 * 24).

const dateDifferenceInDays = (dateInitial, dateFinal) =>
  (dateFinal - dateInitial) / 86_400_000;

dateDifferenceInDays(
  new Date('2017-12-13'),
  new Date('2017-12-22')
); // 9

Date difference in weekdays

Calculating the date difference in weekdays is a bit more involved. We first need a way to determine if a given date is a weekday. Then, we need to create an array of dates between the two dates and filter out the weekends. This can be done using Array.from() and a function to add days to a date. Finally, we can use Array.prototype.filter() to filter out the weekends and return the length of the resulting array.

const isWeekday = date => date.getDay() % 6 !== 0;
const addDaysToDate = (date, n) => {
  const d = new Date(date);
  d.setDate(d.getDate() + n);
  return d;
};

const dateDifferenceInWeekdays = (startDate, endDate) =>
  Array
    .from({ length: (endDate - startDate) / 86_400_000 })
    .filter((_, i) => isWeekday(addDays(startDate, i + 1)))
    .length;

dateDifferenceInWeekdays(
  new Date('Oct 05, 2020'),
  new Date('Oct 06, 2020')
); // 1
dateDifferenceInWeekdays(
  new Date('Oct 05, 2020'),
  new Date('Oct 14, 2020')
); // 7

Date difference in weeks

Calculating the date difference in weeks is still possible with simple date subtraction. We need only divide the difference in milliseconds by the number of milliseconds in a week (1000 * 60 * 60 * 24 * 7).

const dateDifferenceInWeeks = (dateInitial, dateFinal) =>
  (dateFinal - dateInitial) / 604_800_000;

dateDifferenceInWeeks(
  new Date('2023-01-01'),
  new Date('2023-01-08')
); // 1

Date difference in months

Months and years are easier to calculate with other Date methods, instead of subtraction. In such cases, we prefer to use Date.prototype.getFullYear() and Date.prototype.getMonth() to calculate the difference between the two dates.

const dateDifferenceInMonths = (dateInitial, dateFinal) =>
  Math.max(
    (dateFinal.getFullYear() - dateInitial.getFullYear()) * 12 +
      dateFinal.getMonth() -
      dateInitial.getMonth(),
    0
  );

dateDifferenceInMonths(
  new Date('2017-12-13'),
  new Date('2018-04-29')
); // 4

Date difference in years

Calculating the date difference in years is similar to the previous example, but we only need to divide the difference in months by 12.

const dateDifferenceInYears = (dateInitial, dateFinal) =>
  dateDifferenceInMonths(dateInitial, dateFinal) / 12;

dateDifferenceInYears(
  new Date('2017-12-13'),
  new Date('2019-12-15')
); // 2