How can I add minutes, hours or days to a JavaScript date?

You can't get too far in web development without stumbling upon a situation where you have to manipulate dates. Perhaps you need to display a delivery date or a deadline. As easy as this sounds, JavaScript doesn't provide a particularly friendly API for such tasks. But that's nothing that you can't solve with a little ingenuity.

Add seconds to date

The Date object has Date.prototype.getTime() and Date.prototype.setTime() built-in. These two methods allow us to get the current time in a date object and set the time in a date object, respectively.

All values are in milliseconds, so we'll have to multiply the number of seconds by 1000 before setting the time. Note, however, that setting the time mutates the object, so we'll also have to create a new Date object to avoid mutating the original.

const addSecondsToDate = (date, n) => {
  const d = new Date(date);
  d.setTime(d.getTime() + n * 1000);
  return d;
};

addSecondsToDate(new Date('2020-10-19 12:00:00'), 10);
// 2020-10-19 12:00:10
addSecondsToDate(new Date('2020-10-19 12:00:00'), -10);
// 2020-10-19 11:59:50

Add minutes to date

Similarly to seconds, we can add minutes to a date by multiplying the number of milliseconds in a minute (1000 * 60) before setting the time.

const addMinutesToDate = (date, n) => {
  const d = new Date(date);
  d.setTime(d.getTime() + n * 60_000);
  return d;
};

addMinutesToDate('2020-10-19 12:00:00', 10);
// 2020-10-19 12:10:00
addMinutesToDate('2020-10-19 12:00:00', -10);
// 2020-10-19 11:50:00

Add hours to date

To add hours to a date, we can multiply the number of milliseconds in an hour (1000 * 60 * 60) before setting the time.

const addHoursToDate = (date, n) => {
  const d = new Date(date);
  d.setTime(d.getTime() + n * 3_600_000);
  return d;
};

addHoursToDate('2020-10-19 12:00:00', 10);
// 2020-10-19 22:00:00
addHoursToDate('2020-10-19 12:00:00', -10);
// 2020-10-19 02:00:00

Add days to date

For larger values, such as days, we can use Date.prototype.getDate() and Date.prototype.setDate(), instead.

const addDaysToDate = (date, n) => {
  const d = new Date(date);
  d.setDate(d.getDate() + n);
  return d;
};

addDaysToDate('2020-10-15', 10);
// 2020-10-25
addDaysToDate('2020-10-15', -10);
// 2020-10-05

Add weekdays to date

Weekday calculations are a little more involved. We will use the previous code snippet to add days, as needed.

Before we do so, we will use Math.sign() to determine if we should add (+1) or subtract (-1) days. Then, we will use Array.from() to create an array with length equal to the absolute value of n.

Finally, we will use Array.prototype.reduce() to iterate over the array, starting from date and incrementing, using addDaysToDate(). If a day is not a weekday, we will add or subtract another day, as needed, until we find a weekday.

const isWeekday = date => date.getDay() % 6 !== 0;

const addWeekDays = (date, n) => {
  const s = Math.sign(n);
  const d = new Date(date);
  return Array.from({ length: Math.abs(n) }).reduce((currentDate) => {
    currentDate = addDaysToDate(currentDate, s);
    while (!isWeekday(currentDate))
      currentDate = addDaysToDate(currentDate, s);
    return currentDate;
  }, d);
};

addWeekDays('2020-10-05', 5);
// 2020-10-12
addWeekDays('2020-10-05', -5);
// 2020-09-28