Develop Reference

Javascript Date parsing returning strange results in Chrome

I observed some strange Date behaviour in Chrome (Version 74.0.3729.131 (Official Build) (64-bit)).
Following javascript was executed in the Chrome Dev Console:
new Date('1894-01-01T00:00:00+01:00')
// result: Mon Jan 01 1894 00:00:00 GMT+0100 (Central European Standard Time)
new Date('1893-01-01T00:00:00+01:00')
// result: Sat Dec 31 1892 23:53:28 GMT+0053 (Central European Standard Time)
I have already read about non standard date parsing via the Date ctor in different browsers, although providing valid ISO8601 values.
But this is more than strange o_o
In Firefox (Quantum 66.0.3 (64-Bit)) the same calls result in expected Date objects:
new Date('1894-01-01T00:00:00+01:00')
// result: > Date 1892-12-31T23:00:00.000Z
new Date('1893-01-01T00:00:00+01:00')
// result: > Date 1893-12-31T23:00:00.000Z
Is this a bug in Chrome?
My input is valid ISO8601 i guess?
The most important question is, how do I fix this? (hopefully without parsing the input string myself)

Okay, seems like this behaviour cannot be avoided, so you should parse dates manually. But the way to parse it is pretty simple.
If we are parsing date in ISO 8601 format, the mask of date string looks like this:
<yyyy>-<mm>-<dd>T<hh>:<mm>:<ss>(.<ms>)?(Z|(+|-)<hh>:<mm>)?
1. Getting date and time separately
The T in string separates date from time. So, we can just split ISO string by T
var isoString = `2019-05-09T13:26:10.979Z`
var [dateString, timeString] = isoString.split("T")
2. Extracting date parameters from date string
So, we have dateString == "2019-05-09". This is pretty simple now to get this parameters separately
var [year, month, date] = dateString.split("-").map(Number)
3. Handling time string
With time string we should make more complex actions due to its variability.
We have timeString == "13:26:10Z"
Also it's possible timeString == "13:26:10" and timeString == "13:26:10+01:00
var clearTimeString = timeString.split(/[Z+-]/)[0]
var [hours, minutes, seconds] = clearTimeString.split(":").map(Number)
var offset = 0 // we will store offset in minutes, but in negation of native JS Date getTimezoneOffset
if (timeString.includes("Z")) {
// then clearTimeString references the UTC time
offset = new Date().getTimezoneOffset() * -1
} else {
var clearOffset = timeString.split(/[+-]/)[1]
if (clearOffset) {
// then we have offset tail
var negation = timeString.includes("+") ? 1 : -1 // detecting is offset positive or negative
var [offsetHours, offsetMinutes] = clearOffset.split(":").map(Number)
offset = (offsetMinutes + offsetHours * 60) * negation
} // otherwise we do nothing because there is no offset marker
}
At this point we have our data representation in numeric format:
year, month, date, hours, minutes, seconds and offset in minutes.
4. Using ...native JS Date constructor
Yes, we cannot avoid it, because it is too cool. JS Date automatically match date for all negative and too big values. So we can just pass all parameters in raw format, and the JS Date constructor will create the right date for us automatically!
new Date(year, month - 1, date, hours, minutes + offset, seconds)
Voila! Here is fully working example.
function convertHistoricalDate(isoString) {
var [dateString, timeString] = isoString.split("T")
var [year, month, date] = dateString.split("-").map(Number)
var clearTimeString = timeString.split(/[Z+-]/)[0]
var [hours, minutes, seconds] = clearTimeString.split(":").map(Number)
var offset = 0 // we will store offset in minutes, but in negation of native JS Date getTimezoneOffset
if (timeString.includes("Z")) {
// then clearTimeString references the UTC time
offset = new Date().getTimezoneOffset() * -1
} else {
var clearOffset = timeString.split(/[+-]/)[1]
if (clearOffset) {
// then we have offset tail
var negation = timeString.includes("+") ? 1 : -1 // detecting is offset positive or negative
var [offsetHours, offsetMinutes] = clearOffset.split(":").map(Number)
offset = (offsetMinutes + offsetHours * 60) * negation
} // otherwise we do nothing because there is no offset marker
}
return new Date(year, month - 1, date, hours, minutes + offset, seconds)
}
var testDate1 = convertHistoricalDate("1894-01-01T00:00:00+01:00")
var testDate2 = convertHistoricalDate("1893-01-01T00:00:00+01:00")
var testDate3 = convertHistoricalDate("1894-01-01T00:00:00-01:00")
var testDate4 = convertHistoricalDate("1893-01-01T00:00:00-01:00")
console.log(testDate1.toLocaleDateString(), testDate1.toLocaleTimeString())
console.log(testDate2.toLocaleDateString(), testDate2.toLocaleTimeString())
console.log(testDate3.toLocaleDateString(), testDate3.toLocaleTimeString())
console.log(testDate4.toLocaleDateString(), testDate4.toLocaleTimeString())
Note
In this case we are getting Date instance with all its own values (like .getHours()) being normalized, including timezone offset. The testDate1.toISOString will still return weird result. But if you are working with this date, it will probably 100% fit your needings.
Hope that helped :)

This might be the case when all browsers follow their own standards for encoding date formats (but I am not sure on this part). Anyways a simple fix for this is to apply the toISOString method.
const today = new Date();
console.log(today.toISOString());

Getting same unix time stamp for two different times in javascript

I had two ical format timestamps and I want to convert them to normal time first and then to unix time.
Here this is the function I've been using to convert normal time to unix timestamp:
var normal_to_unix = function (date_string) {
var date = new Date(date_string);
return date.getTime() / 1000;
}
This function is fine since date is already in UTC and I need not do any conversions.
Now this is the function I've been using to convert ical time to unix time. The ical time in my case is like "20180603T150000Z".
var ics_to_unix = function (ics_string) {
var year = ics_string.slice(0, 4);
var month = ics_string.slice(4, 6);
var date = ics_string.slice(6, 8);
var hours = ics_string.slice(9, 11);
var minutes = ics_string.slice(11, 13);
var seconds = ics_string.slice(13, 15);
var milliseconds = 0;
console.log(year, month, date, hours, minutes, seconds, milliseconds); // This is example output 2018 06 03 15 00 00 0
return normal_to_unix((new Date(year, month, date, hours, minutes, seconds, milliseconds)).toDateString())
}
Now the problem is I'm getting the same unix time for "20180603T150000Z" and "20180603T160000Z" which are supposed to give different timestamps and it is 1530576000 for both of them.
Is there anything that I'm missing ? Thanks in advance.
Please have a look at this for live example

Several points here:
The toDateString() method returns the date portion of a Date object in human readable form in American English. For your example it is `Tue Jul 03 2018', perhaps that is not what you want.
new Date creates date in your local timezone, which could play well if you use it together with toString(), which will also return the string for date in your local timezone. But it will be subject to daylight saving changes, so I'd avoid using that method.
Another thing I'd like to avoid converting back and forth between strings and dates, since it does a lot of unnecessary computations.
I'd suggest to use the following:
var ics_to_unix = function (ics_string) {
var year = parseInt(ics_string.slice(0, 4));
var month = parseInt(ics_string.slice(4, 6)) - 1; // Jan is 0
var date = parseInt(ics_string.slice(6, 8));
var hours = parseInt(ics_string.slice(9, 11));
var minutes = parseInt(ics_string.slice(11, 13));
var seconds = parseInt(ics_string.slice(13, 15));
return Date.UTC(year, month, date, hours, minutes, seconds) / 1000;
}
I have added explicit conversion of strings to numbers, adjusted the month to match what is used in javascript and also removed the extra call.

How do I calculate the number of days between a particular date in the future and today?

This is what I have tried. Am getting 21 which is definitely not right. What am I doing wrong?
var today = new Date();
var TrumpsStateVisit = new Date('March 12, 2018 12:00:00');
var daysTillTrumpsStateVisit = TrumpsStateVisit.getTime() - today.getTime();
daysTillTrumpsStateVisit = (daysTillTrumpsStateVisit / 864000000); //number of milleseconds in a day
var $countDownTillTrumpsChinaTrip = ('#countDownTillTrumpsChinaTrip');
$countDownTillTrumpsChinaTrip.textContent = Math.floor(daysTillTrumpsStateVisit);

The main reason this doesn't work is that the number of milliseconds in a day is 100*60*60*24=86400000. You also should construct the date differently as that will be March 12, 2018 12:00 in whatever timezone the users browsers in which might not be what you intend.
However there a number of settle issues related to time changes and other issues when subtracting dates so you should really use a library like https://momentjs.com/ that specializes in handling dates and times.

You were so close!
The main problem is that there aren't 864000000 milliseconds in a day, there are only 86400000; you had one zero too many.
A secondary problem occurs with Daylight Savings; not every day has exactly 24 hours in it, so you need to round the result rather than floor it:
var today = new Date();
var TrumpsStateVisit = new Date('March 12, 2018 12:00:00');
var daysTillTrumpsStateVisit = TrumpsStateVisit.getTime() - today.getTime();
daysTillTrumpsStateVisit = (daysTillTrumpsStateVisit / 86400000); //number of milleseconds in a day
console.log(Math.round(daysTillTrumpsStateVisit));
Hope this helps! :)

How do I subtract two dates?

I have a date stored in a database as a string. It looks like this:
Tue Aug 23 2016 00:00:00 GMT-0500 (CDT)
I basically want to tell if today's date is before or after the date in the database.
The code below should sort of explain what I want. The problem is that after the difference variable doesn't return a number variable, which is what I need.
var expire = value.vaccines;
var today = new Date();
var difference = today-expire;
if(difference <= 0){
$(element).css({"color": "#0040ff"});
}
Any ideas on how to subtract these two dates and get a number value?

Assuming both your objects are Date
Although you only require the returned value in milliseconds, I've added the extra step of formatting the value.
Math.floor((today - expire) / (1000*60*60*24))
Taken from Here

You can just you can calculate the difference between the two Date objects and get the absolute value with Math.abs():
var today = new Date(),
expire = value.vaccines,
difference = Math.abs(today - expire); // difference in milliseconds
if (difference <= 0) {
$(element).css({
"color": "#0040ff"
});
}
Check that expire is a valid Date object.

javascript - UTC date object manipulation

The UTC thing is really making me crazy... I am trying to have date and time on site in UTC so it has no affect of any timezone.
What I do, I create a date object
var d = new Date();
//convert it to utc
var utc = d.getTime() + (d.getTimezoneOffset() * 60000);
var utc_date = new Date(utc);
utc_date.setHours(20,0,0)
console.log(utc_date.getTime()) // I want this to be same irrespective of timezone, but don't know why it is changing
Please guide where I am doing wrong..?
UPDATED:
I wanted to create a dropdown of time like on http://jsfiddle.net/HNyj5/ the concept here is I use a timestamp either from client side of selected date or from db and then I generate this dropdown dynamically. So I want the timestamp to be similar on both server/client thats why I am trying to use UTC date object.

You can retrieve the UTC datetime from local time like this (example timezone = GMT+0100):
var currentUTC = new Date; //=>Mon Mar 18 2013 13:53:24
currentUTC.setMinutes(currentUTC.getMinutes()+currentUTC.getTimezoneOffset();
//=> currentUTC now: Mon Mar 18 2013 12:54:06
//or
var someUTC = new Date('1998/03/18 13:52'); //=> Wed Mar 18 1998 13:52:00
someUTC.setMinutes(currentUTC.getMinutes()+currentUTC.getTimezoneOffset();
//=> someUTC now: Wed Mar 18 1998 12:52:00
Or as a Date Extension with a one liner:
Date.prototype.UTCFromLocal = function(){
var a;
return new Date(Date.prototype.setMinutes
.call(a = this,a.getMinutes()+a.getTimezoneOffset()));
}
// usage (current date and time = Mon Mar 18 2013 14:08:14 GMT+0100
var d = new Date().UTCFromLocal(); //=> Mon Mar 18 2013 13:08:14
And to retrieve (from a UTC datetime) you could use:
Date.prototype.LocalFromUTC = function(){
var a;
return new Date(Date.prototype.setMinutes
.call(a = this,a.getMinutes()-a.getTimezoneOffset()));
}

Please guide where I am doing wrong..?
You are building a utc_date that is a completely different time, somehow biased by the getTimezoneOffset. Just do
var d = new Date();
d.getTime(); // milliseconds since epoch
or
Date.now();
And if you're working in UTC-land, you should use d.setUTCHours instead of the local-timezone-dependent setHours.

Actually what I was expecting the JS to do was if I pass the timestamp in the Date constructor it should make object w.r.t that timestamp but it converts it to localtimezone which was making issues for me.
So what I did for solving this problem I get the date object by passing the string of the selected date.
var date = new Date(selected_date_str); rather than passing the timestamp
as I was making dropdown of time with UTC timestamp as its value. The start hour:min of dropdown was dynamic, which I was passing as argument in the function, it was from_hr like if I want to create dropdown of time from 20:00 then I pass from_hr = 20
so now I set hour for the selected date
date.setHours(from_hr, 0, 0);
then I made a utc_time variable for making the the value for dropdown
var utc_time = Date.UTC(date.getFullYear(), date.getMonth(), date.getDate(), from_hr, 0, 0, 0);
this will retain in all timezones, this is what I am going to use as the base. Then in the loop I was adding 15 mins in the time
var count = 0;
jQuery(elem).html('');
while(count <= 95){
var option = '<option value="{0}">{1}:{2}</option>'.format(utc_time/1000, ('0' + date.getHours()).slice(-2), ('0' + date.getMinutes()).slice(-2)); //here i used a format prototype, which you can find in the jsfiddle link of my question
jQuery(elem).append(option);
utc_time += 15 * 60 * 1000; //adding 15 mins in the utc timestamp
date.setMinutes(date.getMinutes() + 15)
count++; }
I was dividing the utc_time with 1000 to make it php compatible, because I was going to retrieve value from here and save in db.