I use the javascript function of the Date class toLocaleDateString() in a project to get the proper formatting according to the user's locale, so simple. However, the day of the month is not always right (It is somethimes one day less than expected). I cannot seem to understand why this happens.

I built a jsfiddle to show the problem. It occurs with the month of october. The date should be 3 like in all the other months but somehow it's 2.

/

Here is the important part of code and the result (My locale is 'de'):

for(var i = 0; i < 12; i++)
{
    var d = new Date(1993,i,3);
    var n = d.toLocaleDateString();
    document.getElementById("demo").innerHTML += n + "<br>";
}

3.1.1993
3.2.1993
3.3.1993
3.4.1993
3.5.1993
3.6.1993
3.7.1993
3.8.1993
3.9.1993
2.10.1993
3.11.1993
3.12.1993

Do you guys have an idea about what's happening here and what can I do to solve it?

Thank you very much!

I use the javascript function of the Date class toLocaleDateString() in a project to get the proper formatting according to the user's locale, so simple. However, the day of the month is not always right (It is somethimes one day less than expected). I cannot seem to understand why this happens.

I built a jsfiddle to show the problem. It occurs with the month of october. The date should be 3 like in all the other months but somehow it's 2.

https://jsfiddle/vincepunkrock/nh1u3ord/

Here is the important part of code and the result (My locale is 'de'):

for(var i = 0; i < 12; i++)
{
    var d = new Date(1993,i,3);
    var n = d.toLocaleDateString();
    document.getElementById("demo").innerHTML += n + "<br>";
}

3.1.1993
3.2.1993
3.3.1993
3.4.1993
3.5.1993
3.6.1993
3.7.1993
3.8.1993
3.9.1993
2.10.1993
3.11.1993
3.12.1993

Do you guys have an idea about what's happening here and what can I do to solve it?

Thank you very much!

• javascript
• string
• date
• locale

• 2 When did "summer time" (or "daylight savings time") end in 1993 in your locale? – Pointy Commented Jan 2, 2017 at 20:24
• It was on sept. 26th. Indeed, when I set the date to the 27th, I get the 26th for the months of september and october. You were on the right track! – Vincent Gagnon Commented Jan 2, 2017 at 20:52
• 1 "…to get the proper formatting according to the user's locale…". There's no guarantee that will happen, since toLocaleDateString is entirely implementation dependent and some implementations seem to ignore user settings entirely. – RobG Commented Jan 3, 2017 at 1:34

3 Answers 3

On Firefox (60) this doesn't show up when initializing the date from numbers (my timezone is GMT-3: observe how the Date initializer adds 3 hours automatically):

>> var d;
>> d = new Date(2019,10,17);
Date 2019-11-17T03:00:00.000Z
>> console.log(d.toLocaleDateString('de'))
17.11.2019

However, when initializing it from a string, problems show up:

>> d = new Date("2019-10-17");
Date 2019-10-17T00:00:00.000Z
>> console.log(d.toLocaleDateString('de'))
16.11.2019

What I found out could solve the problem was manually specifying the timezone as UTC:

>> console.log(d.toLocaleDateString('de', {timeZone: "UTC"}))
17.10.2019

I think Pointy nailed it with his ment. toLocalDateString is definitely factoring in DST with a presumed hour of 00:00:0000 since it's not being specified so the "fall back" hour goes into the prior day. I'm not sure what your locale is to have that only happen in one month but I'm not a pro on time zones. The script works perfectly for me.

Update: set the "time" in your new date to the middle of the day if this isn't the behavior you want.. then the Daylight savings time adjustment won't kick back and forth into different dates..

var d = new Date(1993,i,3, 12,30,30,30);

Works for me with french Language first. It seems the language settings of your browser affects the results in a strange manner. Check your browser language settings.