Date difference when year is smaller than zero

Question

If I'm executing this:

var date = new Date("10.31");
date.setFullYear(-125);

the output of date is Sun Oct 31 -125 00:00:00 GMT+0200 (W. Europe Summer Time)

If I check this on wolframalpha the day seems to be tuesday.

Can someone explain why not the same day is displayed by both source?

And what made you expect that? Do you know how JavaScript handles dates, and what date ranges it accepts? — Racil Hilan, Oct 31 '17 at 13:08
@RacilHilan according to [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date) the range is `The JavaScript Date object range is -100,000,000 days to 100,000,000 days relative to 01 January, 1970 UTC` thats enough for some years less than zero — kevinSpaceyIsKeyserSöze, Oct 31 '17 at 13:17
you should maybe use the explicit date constructor: `new Date(-125, 9, 31);`. Still output Sunday, but that may be a more suitable long-term approch — Apolo, Oct 31 '17 at 13:20
@Apolo thanks for the hint. It's just something I came across. If I'm working with dates in js I just use ISO strings. — kevinSpaceyIsKeyserSöze, Oct 31 '17 at 13:21
Exactly, that's wonderful! And since you are on that page, did you read how you use it with string values? And did you read the warning *"parsing of date strings with the Date constructor is strongly discouraged due to browser differences and inconsistencies"*? — Racil Hilan, Oct 31 '17 at 13:21
No? OK, so when you use `new Date("10.31")`, what do you expect the result to be? — Racil Hilan, Oct 31 '17 at 13:23
@RacilHilan It does not matter I would never create a date like that in js. For the question it fits. — kevinSpaceyIsKeyserSöze, Oct 31 '17 at 13:24
OK, so your question is only about the negative years. My first comment is still valid. You need to understand how JavaScript handles dates. See my answer below. The accepted answer is incorrect. — Racil Hilan, Oct 31 '17 at 17:23

score 2 · Accepted Answer · answered Oct 31 '17 at 17:21

The reason for the difference between JavaScript and wolframalpha website is that JavaScript is calculating the years mathematically, so it includes the year zero. Try to set the year to zero in JavaScript and you will see that it works. However, there is no such a thing as year zero, and the year before year 1 is year 1 BC. Try to set the year to zero on wolframalpha website and you get an error, while it automatically converts all negative years to years BC. This is the correct behavior.

To get the BC years in JavaScript, add 1 to every year below 1. So year 0 becomes 1BC, and year -125 becomes 126BC. In JavaScript this gives you Sunday, and 126BC on wolframalpha website gives you Sunday too. 125BC gives you Tuesday on wolframalpha website, and -124 gives you the same in JavaScript.

var date = new Date();
date.setFullYear(-124);
date.setMonth(9);
date.setDate(31);
console.log(date.toString());
date.setFullYear(-125);
console.log(date.toString());

`add 1 to every year below 1. So year 0 becomes 1BC, and year -125 becomes 126BC` if I add one year it will be -124 — kevinSpaceyIsKeyserSöze, Nov 02 '17 at 08:54
No, I meant adding 1 to the absolute value, so `125+1`, not `-125+1` :). I said *"-124 becomes 125BC"*, not `-125`. The reason is that date values are always positive even for BC dates. There is no year zero and there are no negative years. — Racil Hilan, Nov 03 '17 at 18:56

Luca De Nardi · Answer 2 · 2017-10-31T13:13:38.667

0

Javascript dates start in 1970.

Let's do a quick count.

(new Date()).setYear(-125); //returns -66085584766591 (milliseconds from time 0)
//Let's convert those milliseconds in years...
//-66085584766591 = -2095,56 (years???)

As you can see, you can't rely on negative dates in Javascript.

edited Oct 31 '17 at 13:13

answered Oct 31 '17 at 13:11

Luca De Nardi

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They start in 1980? Maybe 1-1-1970? – Racil Hilan Oct 31 '17 at 13:13
Date starts in 1970 (Unix epoch): `new Date(0) => Thu Jan 01 1970 01:00:00 GMT+0100 (Paris, Madrid)` – Apolo Oct 31 '17 at 13:13
@RacilHilan my bad, hit the wrong number :) corrected. – Luca De Nardi Oct 31 '17 at 13:13
This isn't how javascript negative years works, they produce a BC date. – bryan60 Oct 31 '17 at 13:14
They might produce a BC date but it isn't reliable for sure. That's just what I'm saying – Luca De Nardi Oct 31 '17 at 13:15
1

And what’s the issue with the result? 1970 + 125 = 2095, that’s quite correct. – jcaron Oct 31 '17 at 16:48
Your claim that negative dates in JavaScript are unreliable is incorrect. Your calculation for the negative value `-66085584766591` equally applies to positive values `66085584766591`, then all dates in JavaScript are unreliable. How did you calculate the value 2095.56? You just did a simple math, right? You forgot that the year is not exactly 365 days, didn't you? JavaScript takes the lap years into consideration when it does the calculation. – Racil Hilan Oct 31 '17 at 17:34

score 0 · Answer 3 · answered Oct 31 '17 at 13:13

0

negative years in javascript do produce a BC date but it's kind of a poor design. Wolfram Alpha is probably correct. See this answer for more: Why does Date accept negative values?

answered Oct 31 '17 at 13:13

bryan60

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Date difference when year is smaller than zero

3 Answers3