Get the Number of Hours between 2 Dates in JavaScript

Get the Number of Hours between 2 Dates in JavaScript #

To get the number of hours between 2 dates:

1. Get the number of milliseconds between the unix epoch and the Dates.
2. Subtract the milliseconds of the start date from the milliseconds of the end date.
3. Divide the result by the number of milliseconds in an hour - 1000 * 60 * 60.

Copied!
function getHoursDiff(startDate, endDate) {
  const msInHour = 1000 * 60 * 60;

  return Math.round(Math.abs(endDate - startDate) / msInHour);
}

// 👇️ (9 hours - 6 hours = 3 hours)
console.log(
  getHoursDiff(
    new Date(2022, 1, 24, 6, 44, 0),
    new Date(2022, 1, 24, 9, 45, 30),
  ),
);

// 👇️ (1 day - 0 hours = 24 hours)
console.log(
  getHoursDiff(
    new Date(2022, 1, 23, 9, 40, 0),
    new Date(2022, 1, 24, 9, 45, 0),
  ),
);

The parameters we passed to the Date() constructor in the examples are - year, month (0 = January, 1 = February, etc), day of the month, hour, minutes and seconds.

We created a reusable function that returns the number of hours between 2 dates.

The msInHour variable stores the number of milliseconds there are in an hour.

We could have called the getTime() method on both of the Date objects to achieve the same result.

Copied!
function getHoursDiff(startDate, endDate) {
  const msInHour = 1000 * 60 * 60;

  return Math.round(
    Math.abs(endDate.getTime() - startDate.getTime()) / msInHour,
  );
}

// 👇️ (9 hours - 6 hours = 3 hours)
console.log(
  getHoursDiff(
    new Date(2022, 1, 24, 6, 44, 0),
    new Date(2022, 1, 24, 9, 45, 30),
  ),
);

// 👇️ (1 day - 0 hours = 24 hours)
console.log(
  getHoursDiff(
    new Date(2022, 1, 23, 9, 40, 0),
    new Date(2022, 1, 24, 9, 45, 0),
  ),
);

console.log(new Date().getTime()); // 👉️ 164243142779

In the example, we called the getTime() method to explicitly convert the Date object to a timestamp when subtracting.

This might be the more clear and explicit approach for readers of your code.

We used the Math.abs function to handle a scenario where we subtract a greater timestamp from a smaller one.

The Math.abs function returns the absolute value of a number. In other words, if the number is positive, the number is returned and if the number is negative, the negation of the number is returned.

Copied!
console.log(Math.abs(-3)); // 👉️ 3
console.log(Math.abs(3)); // 👉️ 3

We passed the value to the Math.round function to round to the nearest integer.

Here are some examples of how the Math.round function works.

Copied!
console.log(Math.round(2.49)); // 👉️ 2
console.log(Math.round(2.5)); // 👉️ 3

The function rounds the number up or down to the nearest integer.

If the number is positive and its fractional portion is less than 0.5, it gets rounded to the lower absolute value.