CODEWITHSUNDEEP
javascriptnode.jsdate
Alicia Sykes
Alicia Sykes

Reputation: 7107

Calculate number of days between two dates. Unexpected results

I'm writing a function that calculates how many days ago a given date was from today. (e.g. yesterday = 1, last week = 7, today = 0, tomorrow = -1 and so on)

Seemed simple enough, and using the JavaScript Date() function I initially wrote this: 

let historicalDate = new Date(2017,05,17).getTime(); // example date: last week
let diff = Math.round((new Date().getTime() - historicalDate) / (24*60*60*1000) );

After I got some weird results, I neatened up the code, but still got the same issue, as follows:

/**
* Returns an integer, representing the number of days since a given date
**/
function getNumDaysFromDate(historicalDate){
  const day = 24*60*60*1000;              // The number of milliseconds in one day
  const now = new Date().getTime();       // The time right now 
  const then = historicalDate.getTime();  // The time comparing to
  return Math.round((now - then) / day ); // Find difference in milliseconds, then days
}

// Test1: last week, should return 7
let creationDate1 = new Date(2017,05,17);
console.log("Last week:", getNumDaysFromDate(creationDate1)); // Fail, prints -23

// Test2: yesterday, should return 1
let creationDate2 = new Date(2017,05,23);
console.log("Yesterday:", getNumDaysFromDate(creationDate2)); // Fail, prints -29

// Test3: Today, should return 0
let creationDate3 = new Date();
console.log("Today:", getNumDaysFromDate(creationDate3)); // Pass, prints 0

// Test4: day affer tomrrow, should return -2
let creationDate4 = new Date(2017,05,26);
console.log("Future:", getNumDaysFromDate(creationDate4)); // Fail, prints -32

All the above results appear to be all about 1 month out, (except for 'test 3', today).

I'm sure there is an obvious or simple reason for this, that one of you will spot instantly, but I have spent the last couple of hours mind-blown by it!

Thanks in advance!

Edit: If possible, I'd like to avoid using a library like Moment.js, as this should be possible nativity (?), and is the only date-related calc in my application.

Upvotes: 1

Views: 105

Answers (2)

Apotheka
Apotheka

Reputation: 1

remind the months in JavaScript are zero based (Jan = 0, Feb = 1, ...). So if you need may it's 4 (not 5).

/**
* Returns an integer, representing the number of days since a given date
**/
function getNumDaysFromDate(historicalDate){
  const day = 24*60*60*1000;              // The number of milliseconds in one day
  const now = new Date().getTime();       // The time right now 
  const then = historicalDate.getTime();  // The time comparing to
  var value = Math.round((now - then) / day );
  if(value == 0){
     return 0
    }
  else{
    return value+30;
    }// Find difference in milliseconds, then days
}

// Test1: last week, should return 7
let creationDate1 = new Date(2017,04,17); // 17th of May 2017
console.log("Last week:", getNumDaysFromDate(creationDate1)); // Fail, prints -23

// Test2: yesterday, should return 1
let creationDate2 = new Date(2017,04,23); // 23 of May 2017
console.log("Yesterday:", getNumDaysFromDate(creationDate2)); // Fail, prints -29

// Test3: Today, should return 0
let creationDate3 = new Date();
console.log("Today:", getNumDaysFromDate(creationDate3)); // Pass, prints 0

// Test4: day affer tomrrow, should return -2
let creationDate4 = new Date(2017,04,26); // 26th of May 2017
console.log("Future:", getNumDaysFromDate(creationDate4)); // Fail, prints -32

Upvotes: -3

Rolf Schäuble
Rolf Schäuble

Reputation: 690

Be careful: the Javascript date API is completely insane (exactly like the Java date API).

Month starts with 0 (January) and goes up to 11 (December). So new Date(2017,5,17) actually means June 17th 2017.

Upvotes: 5

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