Javascript dates are weird. Famously, Brendan Eich wrote the first version of Javascript in 10 days - and the function itself was no exception. It is based on code that was . Date ultimately deprecated in Java That means Javascript has inherited a function which was found to be buggy and problematic in Java, leaving it full of issues. You may have even encountered some issues yourself. You may be wondering, then, "what's so weird about it?". Let's look at all the quirks and common pitfalls with Javascript's Date constructor, so you can avoid them. Date Javascript doesn't actually Support Dates It sounds counterintuitive, given the main Javascript date constructor is called Date, but Javascript does not actually support dates. Javascript only supports date times. All Javascript dates are Unix timestamps underneath. That means if we try to create a date, we actually are creating a . All Javascript dates with no time specified default to midnight on that given day. date time let date = new Date(2011, 1, 22); // Notice the date produced has a time attached: // Tue Feb 22 2011 00:00:00 GMT+0000 (Greenwich Mean Time) Parsing Dates Parsing dates as we did above works fine if you know months start at 0, but parsing date strings vary significantly across browsers. It is strongly advised to not parse date strings. Before the ECMAScript 5 specification, how parsed string dates was never defined, and different browsers have many historical quirks that make it very unreliable. Date According to the current specification, only strings conforming to the should be parsable by Javascript, and any other dates should return i.e: ISO-8601 standard NaN let parseMyDate = Date.parse('2022-03-21T11:00:01+00:00'); However, that is not the case. Many browsers allow date parsing outside of this format. This is where it has the potential to get confusing. Let's say you want to parse a date format in standard date format. You take a standard date, and pass it into the function: dd/mm/yyyy parse() let myDate = new Date("5/1/2020"); console.log(myDate); In all modern browsers, this uses the US date format, i.e. - meaning it returns May 1st, not Jan 5th, leading to unexpected results. mm/dd/yyyy Parsing Dates Defaults to UTC Suppose you have a date which has no time or timezone associated with it: let myDate = Date.parse('01 Jan 1999'); console.log(myDate); You might think there is nothing immediately confusing about this - it represents a fixed date in time. However: If your timezone is UTC, this will return . 915148800000 If your timezone is UTC+3:00, this will return , i.e. 3 hours more. 915138000000 If your timezone is UTC-5:00, this will return , i.e. 5 hours less. 915166800000 So if your timezone is west of UTC, for example, , Javascript subtracts 5 hours from the Unix timestamp. Since days start at midnight. -5:00 That means if we try to use this timestamp with a different timezone, for example, in a backend system, we wouldn't get 1st Jan 1999, we get 31st Dec 1998! All of this is because Javascript does not implement dates - every date has a time associated with it - in this case, midnight. Months start at 0 in Javascript Dates If we want to create a date in Javascript we can parse numbers representing year, month and day. For example, if we want to create a date for Feb 22nd, 2011, we'd write this, right? let date = new Date(2011, 2, 22); Only, that gives us . That's because months in Javascript start counting from 0, so February is 1, not 2: Tue Mar 22 2011 00:00:00 GMT+0000 (Greenwich Mean Time) let date = new Date(2011, 1, 22); Incorrect Dates Skip Ahead Let's say you have accidentally created an incorrect date, say 31st Feb, 2022. You pass this into your date function, by mistake, from a database or API: let date = new Date(2022, 1, 31); You might think that this will just return or , but you'd be wrong. Javascript skips to March 3rd! Since February only has 28 days in 2011, and there are 3 extra days, these days are added to the end of the month. In other words, you can't trust to return errors on all incorrect dates. Invalid Date NaN Date Strings are not Parsed to Numbers The weirdest behavior of all is when we don't give Javascript entire strings in parse. For example: let myDate = new Date("0"); console.log(myDate); You might think that this will return the year 0, or perhaps the unix epoch, but it actually returns the year 2000 - . Sat Jan 01 2000 00:00:00 GMT+0000 (Greenwich Mean Time) Even more strangely, though, if we try to increase this, it starts counting in months: console.log(new Date("5")); // Tue May 01 2001 00:00:00 GMT+0100 (British Summer Time) console.log(new Date("11")); // Thu Nov 01 2001 00:00:00 GMT+0000 (Greenwich Mean Time) console.log(new Date("4")); // Sun Apr 01 2001 00:00:00 GMT+0100 (British Summer Time) To top it off, if you try to do , we'll get as the result, since there is no 13th month. new Date("13") Invalid Date Number times are affected by Timezone If we only pass one number to , it will treat it as the Unix timestamp - however, it is not adjusted for timezone. For example, in UTC, the following code returns : new Date() Thu Jan 01 1970 00:00:00 GMT+0000 (Greenwich Mean Time) console.log(new Date(0)); That makes sense, since it's the Unix epoch - however, if we are in UTC-5:00, that code returns - i.e. That means, by default, time zones can lead to a lot of confusion - if we expected the date to be 1st Jan 1970, we immediately have an issue when using method like . Ultimately, we can resolve this by using the method - but this complication leads to a lot of confusion. Wed Dec 31 1969 19:00:00 GMT-0500 (Eastern Standard Time) 5 hours before. Date().toLocaleString() .toUTCString() Years are really inconsistent You might have thought we've gotten off easy, and only timestamps and timezones are broken - but even years are inconsistent. If we wanted to create a date for the 1st Jan, in the year 0, you might think we'd write this: console.log(new Date(0, 0, 0)); Since months start from 0, this looks right - but actually, if the year is less than 100, 0 means the year 1900. Alright, you might think, I suppose this should return 1st Jan 1900 instead - but that's actually wrong too - since days are indexed from 1, not 0. The above code returns - since the 0th day of the month is counted as the last day from the previous month. Here are a few other examples: Sun Dec 31 1899 00:00:00 GMT+0000 (Greenwich Mean Time) console.log(new Date(0, 0, 0)); // Sun Dec 31 1899 00:00:00 GMT+0000 (Greenwich Mean Time) console.log(new Date(50, 0, 0)); // Sat Dec 31 1949 00:00:00 GMT+0000 (Greenwich Mean Time) console.log(new Date(30, 0, 0)); // Tue Dec 31 1929 00:00:00 GMT+0000 (Greenwich Mean Time) console.log(new Date(24, 0, 0)); // Mon Dec 31 1923 00:00:00 GMT+0000 (Greenwich Mean Time) As soon as you get above the year 100, it then does go back to counting the years normally. So the below code actually gives us the year 101, not the year 2001: console.log(new Date(101, 0, 0)); // Fri Dec 31 0100 00:00:00 GMT-0001 (Greenwich Mean Time) This may be useful if you are using years after 1900, but it is incredibly counterintuitive for anything before. Why doesn't anyone fix Javascript Dates? The Javascript Date function is fundamentally broken in many ways - which is why most people use tools like , but Moment.js why hasn't it been fixed? The main reason is that most of the web has been built on code that took into consideration the flaws with Date. As such, changing now would result in many websites simply breaking. To remedy this situation, Javascript is introducing a totally new set of standards called Temporal which will occupy a different namespace than Date, and will solve most of the problems described in this article. Until then, we are stuck with the quirks Javascript Dates produce. Also Published here
