How To Calculate Difference Between Dates In Javascript

Welcome to a tutorial on how to calculate the difference between 2 dates in Javascript. Need to know the difference between 2 dates in hours, days, weeks, or months in Javascript?

To calculate the difference between 2 dates in Javascript:

  1. Parse the dates into date objects.
    • var dateA = new Date("2009-10-11");
    • var dateB = new Date("2011-10-09");
  2. Subtracting the 2 date objects will result in the difference in microseconds.
    • var diff = Math.abs(dateA - dateB);
  3. Finally, just calculate the difference accordingly. For example, 1 day has 24 hrs X 60 mins X 60 secs X 1000 ms = 86400000 ms.
    • var diffDays = Math.floor(diff / 86400000);

That covers the basics, but read on for more examples!

ⓘ I have included a zip file with all the source code at the start of this tutorial, so you don’t have to copy-paste everything… Or if you just want to dive straight in.

 

 

REAL QUICK SLIDES

 

TABLE OF CONTENTS

Download & Notes Date Difference Useful Bits & Links
The End

 

DOWNLOAD & NOTES

Firstly, here is the download link to the example code as promised.

 

EXAMPLE CODE DOWNLOAD

Click here to download all the example source code, I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

QUICK NOTES

If you spot a bug, please feel free to comment below. I try to answer questions too, but it is one person versus the entire world… If you need answers urgently, please check out my list of websites to get help with programming.

 

 

DATE DIFFERENCE

All right, let us now get into the examples of how to calculate the difference between 2 dates in Javascript.

 

1) DIFFERENCE IN HOURS, DAYS, WEEKS

1-diff.html
// (A) PARSE INTO DATE OBJECTS
var dateA = new Date("2012-11-10");
var dateB = new Date("2011-10-09");

// (B) DIFFERENCE IN MICROSECONDS
var diff = Math.abs(dateA - dateB);
console.log("Microseconds: " + diff);

// (C) DIFFERENCE IN SECONDS
diff = Math.floor(diff / 1000);
console.log("Seconds: " + diff);

// (D) DIFFERENCE IN HOURS
// 1 HR = 60 MINS X 60 SECS = 3600 SECS
var diffhrs = Math.floor(diff / 3600);
console.log("Hours: " + diffhrs);

// (E) DIFFERENCE IN DAYS
// 1 DAY = 24 HRS X 60 MINS X 60 SECS = 86400 SECS
var diffdays = Math.floor(diff / 86400);
console.log("Days: " + diffdays);

// (F) DIFFERENCE IN WEEKS
// 7 DAYS = 604800 SECS
var diffweeks = Math.floor(diff / 604800);
console.log("Weeks: " + diffweeks);

For you guys who have not heard of the Unix Timestamp, it is simply the number of seconds that have passed since 1 Jan 1970. In Javascript, we can get the Unix Timestamp (in microseconds) of a date using the getTime() function.

  • var dateA = new Date("2012-11-10");
  • var dateB = new Date("2011-10-09");
  • var unixA = dateA.getTime();
  • var unixB = dateB.getTime();

So far so good? Let’s get back to the example – We can then use the Unix Timestamps to calculate the difference between 2 dates.

  • var diff = Math.abs(unixA - unixB); // MICROSECONDS
  • diff = Math.floor(diff / 1000); // SECONDS

Yep, this is the “long-form”. As you see from the above example, we can directly subtract the date objects themselves Math.abs(dateA - dateB) to get the difference in microseconds. The rest should be pretty straightforward –

  • One hour has 3600 seconds. So to get the difference in hours, we do a simple divide Math.floor(diff / 3600).
  • One day has 86400 seconds, to get the difference in days – Math.floor(diff / 86400)
  • And so on…

 

 

2) DIFFERENCE IN MONTHS

2-months.html
// (A) DATES
var dateA = new Date("2011-01-10");
var dateB = new Date("2010-08-09");

// (B) DIFFERENCE IN YEARS
var diffYears = Math.abs(dateA.getFullYear() - dateB.getFullYear());

// (C) DIFFERENCE IN MONTHS
var diffMonths = (12 * diffYears) + (dateA.getMonth() - dateB.getMonth());

Some of you sharp code ninjas should have noticed by now – One month is not exactly 30 days, the above method of dividing the seconds is not going to be accurate. A little bit more painful in this case, but we can still get the difference by calculating with the formula (12 X ABSOLUTE(DIFFERENCE-IN-YEARS)) + (MONTH-A - MONTH-B).

 

 

USEFUL BITS & LINKS

That’s all for the main tutorial, and here is a small section on some extras and links that may be useful to you.

 

INFOGRAPHIC CHEAT SHEET

Difference Between Dates In Javascript (click to enlarge)

 

LINKS & REFERENCES

 

THE END

Thank you for reading, and we have come to the end. I hope that it has helped you to better understand, and if you want to share anything with this guide, please feel free to comment below. Good luck and happy coding!

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