Develop Reference

How to correctly calculate difference in days between 2 dates with DST change?

I have a task to get days difference between 2 dates. My solution is like here https://stackoverflow.com/a/543152/3917754 just I use Math.ceil instead of Math.round - cause 1 day and something is more than 1 day.
It was fine until I got dates between DST change. For example:
In my timezone DST change was on 30 Oct.
So when I'm trying to find days diff between dates 20 Oct and 10 Nov in result I get 23 instead of 22.
There are solution how to identify DST date but is it good solution to add/substract 1 day if date is/isn't dst?
function datediff(toDate, fromDate) {
const millisecondsPerDay = 1000 * 60 * 60 * 24; // milliseconds in day
fromDate.setHours(0, 0, 0, 0); // Start just after midnight
toDate.setHours(23, 59, 59, 999); // End just before midnight
const millisBetween = toDate.getTime() - fromDate.getTime();
var days = Math.ceil(millisBetween / millisecondsPerDay);
return days;
}
var startDate = new Date('2022-10-20');
var endDate = new Date('2022-11-10');
console.log('From date: ', startDate);
console.log('To date: ', endDate);
console.log(datediff(endDate, startDate));

Count days, hours and minutes to a specific date with javascript

I have this script that counts the days, hours and minutes from the time at the moment until May 8 at 23:59:59 - 1 second before May 9, and displays it as a counter with FlipClock, but for some reason it is missing 2 days and 1 hour. From today March 23 at 10:37 (24 hr clock) there are 48 days, 13 hours, 23 minutes hours, but my clock shows 46 days, 12 hours, 23 minutes.
Should be very simple, but I cannot figure out where the 2 days and 1 hour are gone missing.
I have this javascript:
// Grab the current date
var currentDate = new Date();
// Set the date to May 8
var futureDate = new Date(2016, 04, 08, 23, 59, 59);
// Calculate the difference in seconds between the future and current date
var diff = futureDate.getTime() / 1000 - currentDate.getTime() / 1000;
var clock = new FlipClock($('.clock'), diff, {
clockFace: 'DailyCounter',
countdown: true,
showSeconds: false,
language: 'da'
});

If I have to do anything with times, I usually use momentjs, it does not mean you have to, but I know it works and it works correctly enough for my criterias.
Duration format is not part of moment js, it is a feature. You could roll one yourself, but John Madhavan-Reese has already done this.
var futureDate = moment(new Date(2016, 04, 08, 23, 59, 59));
setInterval(function() {
var ms = moment().diff(moment(futureDate));
var duration = moment.duration(ms).format("yy-MM-dd hh:mm:ss");
$("#time").text(duration);
}, 200);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.12.0/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment-duration-format/1.3.0/moment-duration-format.min.js"></script>
<span id="time"><span>

Javascript: find beginning of Advent weeks each year

I have created the following code (which works) to print something different based on the weeks of a specified month:
<script language="javascript">
<!--
var advent;
mytime=new Date();
mymonth=mytime.getMonth()+1;
mydate=mytime.getDate();
if (mymonth==12 && (mydate >= 1 && mydate <= 6)){document.write("xxx");
}
if (mymonth==12 && (mydate >= 7 && mydate <= 13)){document.write("yyy");
}
if (mymonth==12 && (mydate >= 14 && mydate <= 20)){document.write("zzz");
}
if (mymonth==12 && (mydate >= 21 && mydate <= 30)){document.write("qqq");
}
//-->
</script>
But I need this to change for Advent each year and Advent changes based on when Christmas falls each year:
Advent starts on the Sunday four weeks before Christmas Day. There are
four Sundays in Advent, then Christmas Day. The date changes from year
to year, depending on which day of the week Christmas fall. Thus, in
2010, Advent began on 28 November. In 2011, it will occur on 27
November.
How do I calculate when the weeks of Advent begin each year?

Start with a Date that's exactly 3 weeks before Christmas Eve. Then, walk backwards until the day-of-week is Sunday:
function getAdvent(year) {
//in javascript months are zero-indexed. january is 0, december is 11
var d = new Date(new Date(year, 11, 24, 0, 0, 0, 0).getTime() - 3 * 7 * 24 * 60 * 60 * 1000);
while (d.getDay() != 0) {
d = new Date(d.getTime() - 24 * 60 * 60 * 1000);
}
return d;
}
getAdvent(2013);
// Sun Dec 01 2013 00:00:00 GMT-0600 (CST)
getAdvent(2012);
// Sun Dec 02 2012 00:00:00 GMT-0600 (CST)
getAdvent(2011);
// Sun Nov 27 2011 00:00:00 GMT-0600 (CST)
(2013 and 2012 were tested and verified against the calendar on http://usccb.org/. 2011 was verified against http://christianity.about.com/od/christmas/qt/adventdates2011.htm)

Here's what I was talking about in my comment:
function getAdvent(year) {
var date = new Date(year, 11, 25);
var sundays = 0;
while (sundays < 4) {
date.setDate(date.getDate() - 1);
if (date.getDay() === 0) {
sundays++;
}
}
return date;
}
DEMO: http://jsfiddle.net/eyUjX/1/
It starts on Christmas day, in the specific year. It goes into the past, day by day, checking for Sunday (where .getDate() returns 0). After 4 of them are encountered, the looping stops and that Date is returned.
So to get 2009's beginning of Advent, use: getAdvent(2009);. It returns a Date object, so you can still work with its methods.
As a reference of its methods: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Date

You can get Advent Sunday by adding 3 days to the last Thursday in November, which seems simpler:
function getAdventDay(y){
var advent=new Date();
advent.setHours(0,0,0,0);
//set the year:
if(typeof y!='number')y=advent.getFullYear();
//get the last day of november:
advent.setFullYear(y,10,30);
//back up to the last thursday in November:
while(advent.getDay()!==4)advent.setDate(advent.getDate()-1);
//add 3 days to get Sunday:
advent.setDate(advent.getDate()+3);
return advent;
}
getAdventDay(2013)
/*
Sun Dec 01 2013 00:00:00 GMT-0500 (Eastern Standard Time)
*/

const getFirstAdvent=function(y){
const firstAdvent=new Date(y,11,3);
firstAdvent.setDate(firstAdvent.getDate()-firstAdvent.getDay());
return firstAdvent;
};
alert(getFirstAdvent(2020));

I love these challenges, here's how it can be done with recursion. First I find the fourth Sunday. Then I Just keep minusing 7 days until I have the other 3. The variables firstSunday, secondSunday, thirdSunday and fourthSunday - contains the dates.
EDIT: I believe I misunderstood, but the firstSunday variable Will be the date you are looking for.
Demo
Javascript
var year = 2011;//new Date().getFullYear();
var sevenDays = (24*60*60*1000) * 7;
var foundDate;
var findClosestSunday = function(date){
foundDate = date;
if (foundDate.getDay() != 0)
findClosestSunday(new Date(year,11,date.getDate()-1));
return foundDate;
}
var fourthSunday = findClosestSunday(new Date(year, 11, 23));
var thirdSunday = new Date(fourthSunday.getTime() - sevenDays);
var secondSunday = new Date(fourthSunday.getTime() - sevenDays *2);
var firstSunday = new Date(fourthSunday.getTime() - sevenDays *3);
console.log
(
firstSunday,
secondSunday,
thirdSunday,
fourthSunday
);

Javascript works with time in terms of milliseconds since epoch. There are 1000 * 60 * 60 *24 * 7 = 604800000 milliseconds in a week.
You can create a new date in Javascript that is offset from a know date doing this:
var weekTicks, christmas, week0, week1, week2, week3;
weekTicks = 604800000;
christmas = new Date(2013, 12, 25);
week0 = new Date(christmas - weekTicks);
week1 = new Date(week0 - weekTicks);
week2 = new Date(week1 - weekTicks);
week3 = new Date(week2 - weekTicks);
See how that works for you.
Also, the Date.getDay function will work to help you find which day of the month is the first Sunday.

Javascript adding days to a Date

Hi I am trying to create a variable today that is the current date today. I am trying to add 106 days to it which works successfully. Then I am trying to create a second variable today2 and subtract 31 days from the 'today' variable (current date + 106 -31). This part is not working. This is what it is giving me...
Thu Mar 28 11:52:21 EDT 2013
Tue Nov 27 11:52:21 EST 2012
The second line is not 31 days before the first line. Can someone help me correct this?
Feel free to play with my jsfiddle http://jsfiddle.net/fjhxW/
<div id="current"></div>
<div id="current2"></div>
<div id="current3"></div>
var today = new Date();
var today2 = new Date();
today.setDate(today.getDate() + 106);
today2.setDate(today.getDate() - 31);
var dd = today.getDate();
var mm = today.getMonth(); //January is 0!
var yy = today.getFullYear();
document.getElementById('current').innerHTML = today;
document.getElementById('current2').innerHTML = today2;

it's Xmas time so I give the answer just to copy/paste:
var oneDay = 24 * 60 * 60 * 1000, // 24h
today = new Date().getTime(), // in ms
firstDate,
secondDate;
firstDate = new Date(today + 106 * oneDay);
secondDate = new Date(firstDate.getTime() - 31 * oneDay);

try datejs:
Date.parse('t - 31 d'); // today - 31 days
Date.today().add(106).days().add(-31).days();

You cannot pass a negative number to setDate. setDate is used to set the date to set the absolute day, not relative days.
From the docs:
If the parameter you specify is outside of the expected range, setDate attempts to update the date information in the Date object accordingly. For example, if you use 0 for dayValue, the date will be set to the last day of the previous month.

A mathemathical solution:
Add 75 days to your current day (106 - 31), then add 31 days to that date. Change the order in what you are showing both dates on your code.
Why go forward and backward when you can always go forward?

Calculate Date with start date and number of days with javascript

I am trying to calculate a date from a start date and number of days, so basicly add the number of days to a start date and get an end date. The issue is I get some strange results, but only on one date, I have been a few days now trying to figure this one out.
The function is:
CallculateDateFromDays = function(startDate, days) {
var policy_start_date_array = startDate.split("-");
var policy_start_date = new Date(policy_start_date_array[2], policy_start_date_array[1]-1, policy_start_date_array[0]);
var policy_start_date_add = new Date(policy_start_date.getTime() + ((days-1) * 24 * 60 * 60 * 1000));
var dateString = ("0" + (policy_start_date_add.getDate())).slice(-2) + "-" + ("0" + (policy_start_date_add.getMonth()+1)).slice(-2) + "-" + policy_start_date_add.getFullYear();
return dateString;}
The thing is it works until I use the date "28-10-2012" it gives me back the same date even if I add 2 days.
Any ideas, I am stumped.

Likely your local timezone changes because of the end of the daylight saving time.
> new Date(2012, 9, 28)
Sun Oct 28 2012 00:00:00 GMT+0200
> // 48 hours later:
> new Date(new Date(2012, 9, 28) + 2 * 24 * 60 * 60 * 1000)
Mon Oct 29 2012 23:00:00 GMT+0100
Always use the UTC methods!
BTW, adding days is much more easier with setDate, which also works around timezone issues:
function calculateDateFromDays(startDate, days) {
var datestrings = startDate.split("-"),
date = new Date(+datestrings[2], datestrings[1]-1, +datestrings[0]);
date.setDate(date.getDate() + days);
return [("0" + date.getDate()).slice(-2), ("0" + (date.getMonth()+1)).slice(-2), date.getFullYear()].join("-");
}

On October 28th the time changes from DST to "normal", so the day is not equal 24h. That may cause issues in your code.
Also why (days-1) * 24 * 60 * 60 * 1000? If you set days to 1 the whole expression evaluates to zero...

There's an easier way to achieve that :
http://jsfiddle.net/pjambet/wZEFe/2/

Use javascript date format:
How to format a JavaScript date
So you don't have to manually try to parse date strings.