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Error incrementing days to a specific date

I'm trying to increment one day to a given date. My code, inspired by this answer, looks like:
var date = getDateFromUIControl();
var backupDate = new Date();
backupDate.setDate(date.getDate() + 1);
However, I'm seeing a strange behaviour. Today is December 5th, 2019. If the user selects January 1, 2020 (stored in date variable), then backupDate ends up being January 2nd, 2019, instead of 2020. What is wrong with this code? How should I go about incrementing the date, if what I'm doing is wrong?
Note: because of whatever policies my company has, I can't use any JavaScript library other than jQuery.

new Date() returns the current Date(example: 05/12/2019). You are just changing the date alone in current date. Still the year is 2019.
it should be like,
date.setDate(date.getDate() + 1);
if you can't change the original date object, then it can be done like this,
var changedDate = new Date(date);
changedDate.setDate(changedDate.getDate() + 1);

var date = getDateFromUIControl();
var backupDate = new Date();
backupDate.setDate(new Date(date).getDate() + 1);

nextDay is one day after date:
var date = getDateFromUIControl();
var nextDay = new Date(date.getYear(), date.getMonth(), date.getDate()+1);
Also you don't need to worry about overflowing d.getDate()+1 (e.g. 31+1) - the Date constructor is smart enough to go into the next month.

If 7 or more days past html date and if not paid change the innerHTML using only vanilla JS

If I have the following hard coded HTML date:
<h2 id="date">17/02/2018</h2>
<h2 id="status">Not paid</h2>
With a deadline being the h2 date plus 1 week or more.
How would I write javascript so that if the deadline is gone and the h2 status is not paid then how would I change the <h2 id="status">Not paid</h2>
to display the text overdue instead of Not paid
I tried getting the current date then adding 7 days by using moment.js:
moment().format('L').add(7, 'days'); since it gives me the same date format (dd/mm/yyyy) then comparing it to the h2 date but the problem is since moment.js will always get today's date and add 7 days to it so it will always be 7 days in the future. Not 7 days from the h2 date.
A seemingly simple problem which I have been stuck with for 2 days.

You need to get access to the date first, by pulling the text from the date element:
let myDate = document.getElementById('date').innerText;
Then, you can use moment to instantiate that as a moment date (assuming it's in a valid format):
let momentDate = moment(myDate, "DD/MM/YYYY");
Now you can work with momentDate by adding days to it, comparing it to other dates, etc., e.g.:
let oneWeekLater = momentDate.add(7, 'days');
If you want to change the value of the "status" element, you can then do something like:
let statusElement = document.getElementById("status");
statusElement.innerText = "Thank you for paying!";

Heres the solution:
const currentStatus = document.getElementById('status').innerText;
let myDate = document.getElementById('date').innerText;
//get the date in the html
// 100% correct
let momentDate = moment(myDate, "DD/MM/YYYY");
// gets the myDate variable and converts it into soemthing JS can understand
// 100% correct
let oneWeekLater = momentDate.add(7, 'days').format("YYYY/MM/DD");
// Need to comapare the dates based on bigest number first which is the year then
month etc... it then adds one week on to it
let today = moment(new Date()).format("YYYY/MM/DD");
// prints out todays date in the same format as oneWeekLater so they cna be compared
if(currentStatus.innerText == "Up-paid"){
if (oneWeekLater >= today){
document.write('Overdue')
} else{
document.write('not paid')
}
} else {
currentStatus.innerText = "Thank you for paying"
}

Countdown to a single day each year (and find the date of that day in current year)

I am looking to do something quite complex and I've been using moment.js or countdown.js to try and solve this, but I think my requirements are too complex? I may be wrong. Here is the criteria...
I need to be able to have the following achievable without having to change the dates manually each year, only add it once and have many countdowns on one page.
Find current date
Find current year
Find current month
Find day within week of month that applies
¬ 3rd Sunday or 2nd Saturday
Convert to JS and output as html and run countdown
When past date - reset for following year
Pretty mental. So for example if an event is always on the 3rd Sunday of March. The date would not be the same each year.
2016 - Sunday March 19th
2017 - Sunday March 20th
2018 - Sunday March 18th etc.
I hope this is explained well, I realise it may be a total mess though. I managed to get it resetting each year with the date added manually but then someone threw in the spanner of the date being different each year.
var event = new Date();
event = new Date(event.getFullYear() + 1, 3 - 1, 19);
jQuery('#dateEvent').countdown({ until: event });
<div id="dateEvent"></div>

I have edited this answer as I have now put together a solution that works for me. As I believe this isn't simple coding due to the fact it wasn't actually answered 'Please, this is basic coding. pick up a javascript book and learn to code', yeah thanks...
// get the specific day of the week in the month in the year
function getDay(month) {
// Convert date to moment (month 0-11)
var myMonth = moment("April", "MMMM");
// Get first Sunday of the first week of the month
var getDay = myMonth.weekday(0); // sunday is 0
var nWeeks = 3; // 0 is 1st week
// Check if first Sunday is in the given month
if (getDay.month() != month) {
nWeeks++;
}
// Return 3rd Sunday of the month formatted (custom format)
return getDay.add(nWeeks, 'weeks').format("Y-MM-D h:mm:ss");
}
// print out the date as HTML and wrap in span
document.getElementById("day").innerHTML = '<span>' + getDay() + '</span>';
Using
<script src="moment.js"></script>
Hope it helps someone - I'll update when I figure how to + 1 year after it's checked current date and event has passed. I'll look in that JS book.

Please take a look at the below code, I explained in the comment what what does.
You use it by supplying a javascript Date object of any wished start date, and then add as a second value the corresponding year you wish to know the date in.
var date = new Date("2016-03-20");
function getDayInYear(startDate, year) {
// get a moment instance of the start date
var start = moment(startDate);
// collect the moment.js values for the day and month
var day = start.day();
var month = start.month();
// calculate which week in the month the date is.
var nthWeekOfMoth = Math.ceil(start.date() / 7);
// Build up the new moment with a date object, passing the requested year, month and week in it
var newMoment = moment(new Date(year,month,(nthWeekOfMoth * 7)));
// Return the next instance of the requested day from the current newMoment date value.
return newMoment.day(day);
}
var oldMoment = moment(date);
var newMoment2017 = getDayInYear(date,2017);
var newMoment2018 = getDayInYear(date,2018);
console.log(oldMoment.format('YYYY MMMM dddd DD'));
console.log(newMoment2017.format('YYYY MMMM dddd DD'));
console.log(newMoment2018.format('YYYY MMMM dddd DD'));
/** working from today up to 10 years into the future **/
var date = new Date();
var year = date.getFullYear();
for(var i = 0; i < 11; i++) {
console.log(getDayInYear(date, year+i).format('YYYY MMMM dddd DD'));
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.js"></script>

See if date selected is future from today - JavaScript

I have to compare the date that they want to put in and the current date today, and if they have put in a date that is in the future, then alert them to change the date, otherwise insert the data.
Basically I am having issues comparing the dates. here is my code:
var today = year + '-' + month + '-' + day + ' 00:00:00';
var d1 = new Date(postdate); // postdate = 2014/02/01 ie: 1 Feb 2014
var d2 = new Date(today); // todays date
if(d1>d2){
alert('You cannot post in the future!');
}
But that doesnt seem to work. Where am I going wrong?

Convert the dates into a comparable number, like milliseconds.
if(d1.valueOf()>d2.valueOf()){
alert('You cannot post in the future!');
}

You don't need to create a new variable today.
If by today you are trying to get today's date, you can simply do
var today = new Date();
var d1 = new Date(postdate); // postdate = 2014/02/01 ie: 1 Feb 2014
//----------
var d2 = new Date(year,month,day); // todays date
//----------
if(d1>d2){
alert('You cannot post in the future!');
}
Remember month is 0 based index. So, for december it would be 11.

Compare the dates with the same format, if today is 2014-01-24 00:00:00 then postdate also should be 2014-02-01 00:00:00
Then use + prefix to compare milliseconds:
if(+d1 > +d2){
alert('You cannot post in the future!');
}

Comparing dates Java Script

I have the following scenario that I am strugling to code.
I have a valuation date that is a string that is chosen by a user from a calander popup. What I need to do is take that date and pass it into a function that works outs a second date depending on the value of that date. If the first date is more than 7 days from the first day of the month use the first day of the month else use the last day of the month. This needs to happen in client side as this date need to be displayed after they have chosen the first date.
SO far I have the below:
Function CompareDate()
{ var date1 = document.getElementById("textbox1");
var x = new date();
var year = x.getYear();
var day = x.getDay();
var thisMonthFirstDay = new Date(year, month,1)
var thisMonthLastDate = ....
var 1day = 1000*60*60*24
var date1_ms = recdate
var date2ms = thisMonthFirstDay.gettime()
if(Math.round(difference_ms/1day) > 7
{var textbox = document,getelementbyid("textbox2");
textbox.value = texbox.value + thisMonthLastDate
}
else
{
textbox.value = texbox.value + thisMonthFirstDay }
}
Any examples of how this can be done would be greatly appeciated.
Cheers

getDate() will give you the day of month (e.g. 18), so if (getDate() <= 7) { outputDate = 1; } If you're having a problem getting the last day of each month for the else statement, I generally use a 12 capacity array with hard-coded values, adding 1 to February if (year % 4 == 0).

I have managed to resolve this after a finding the parseDate() function on a fiddler site. That allowed me to convert the date from this format (31 Jan 2013) to a date and then I could just use the getDay(function) to see if the day was > 7. From there it was easy!
Thanks for above suggestions.