How to calculate the difference of days between two dates in JavaScript

Have you ever tried calculating the number of days between two dates in JavaScript without a library? Well, here's how

Short answer:

const firstDate = new Date("05/10/2022") // 10th May, 2022
const secondDate = new Date() // today, 14th May, 2022

const millisecondsDiff = secondDate.getTime() - firstDate.getTime()

const daysDiff = Math.round(
  millisecondsDiff / (24 * 60 * 60 * 60)
)

Long answer

To find the difference between two dates, you first need to get the time value in milliseconds. You get this using the getTime method on the Date instance:

const firstDate = new Date("05/10/2022") // 10th May, 2022
const secondDate = new Date() // today, 14th May, 2022

const firstDateInMs = firstDate.getTime()
const secondDateInMs = secondDate.getTime()

The millisecond value is a number on which you can then perform arithmetic operations like subtractions.

The next step is finding the difference between both dates:

const differenceBtwDates = secondDateInMs - firstDateInMs

console.log(differenceBtwDates)
// 351628869

Now, you have the difference in milliseconds between both dates. This is where you convert the milliseconds to days. How to do that?

First, you get the equivalence of 1 day (24 hours) in milliseconds. Here's how:

const aDayInMs = 24 * 60 * 60 * 1000

console.log(aDayInMs)
// 86400000

• 24 stands for 24 hours
• 60 stands for 60 minutes, which makes an hour, thereby converting 24 hours to minutes
• the next 60 stands for 60 seconds, which makes a minute
• and 1000 stands for 1000 milliseconds, which makes a second

Now that you have the milliseconds value for a day, you can use it to determine how many days the difference between the dates are.

If 86400000 (aDayInMs) milliseconds makes a day, then, 351628869 (differenceBtwDates) milliseconds makes:

const daysDiff = differenceBtwDates / aDayInMs

console.log(daysDiff)
// 4.0782153125

As it returns a floating number, you can round it up with Math.round():

const daysDiff = Math.round(differenceBtwDates / aDayInMs)

console.log(daysDiff)
// 4

Comparing May 10th and May 14th, it is a 4-day difference.

Here's the full code:

const firstDate = new Date("05/10/2022") // 10th May, 2022
const secondDate = new Date() // today, 14th May, 2022

const firstDateInMs = firstDate.getTime()
const secondDateInMs = secondDate.getTime()

const differenceBtwDates = secondDateInMs - firstDateInMs

const aDayInMs = 24 * 60 * 60 * 1000

const daysDiff = Math.round(differenceBtwDates / aDayInMs)

console.log(daysDiff)
// 4

firstDate and secondDate can be any dates at all. I used May 10th and 14th for explanation purposes.