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Javascript: Subtract date to date. Today.getDate() is not a function

I have been trying to subtract date format (yyyy-MM-dd) to another date format but to no avail. I looked up online but it's confusing and I got this error which says that is it not a function for today.getDate(). Could anyone provide me with a solution in subtracting? I am at my wits end.
Thanks in advance.
for (var i = 0; i < 3; i++) {
var startDate = "2020-08-19"
var today = new Date();
var dd = today.getDate();
var mm = today.getMonth() + 1; //January is 0!
var yyyy = today.getFullYear();
if (dd < 10) {
dd = '0' + dd;
}
if (mm < 10) {
mm = '0' + mm;
}
today = yyyy + '-' + mm + '-' + dd;
today.setDate(today.getDate() - startDate);
console.log(today.getDate() - startDate);
if ((startDate - today) >= 30) {
console.log("hello");
} else if ((startDate - today) <= 30) {
console.log("bye");
}
};

You are mixing a few concepts:
var today = new Date();
This creates a Date object that wraps the number of milliseconds from 1/1/1970 at midnight UTC.
today = yyyy + '-' + mm + '-' + dd;
This is creating a string.
If your goal is to find out how many days are between two dates, your best bet is to use milliseconds since Unix epoch, sometimes called "Unix timestamps." With these timestamps, subtracting is straightforward since they're just numbers; you'll get the number of milliseconds between the two points of time. Getting from timestamps to number of days then becomes a matter of division:
const diff = Date.now() - Date.parse("2020-08-19");
const numDays = diff / (1000 * 60 * 60 * 24);

Add future days to Javascript excluding on sunday

I have the following javascript code that allows a user to only select four days in future from today's date.
$(function() {
var dtToday = new Date();
var month = dtToday.getMonth() + 1;
if (dtToday.getDay() === 0) {
var day = dtToday.getDate() + 5;
} else {
var day = dtToday.getDate() + 4;
}
var year = dtToday.getFullYear();
if (month < 10)
month = '0' + month.toString();
if (day < 10)
day = '0' + day.toString();
var maxDate = year + '-' + month + '-' + day;
$('.datepicker').attr('max', maxDate);
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
I would like the code to skip Sunday while incrementing the future days by four. I have tried using if statements but they only check if today's date is sunday while i would like to achieve a situation whereby if any of the future 4 days is Sunday, it should skip it.
Thanks

If today is Wednesday (3), Thursday (4), Friday (5), or Saturday (6), then your 4-day window would include Sunday (hence you should add 1 to days).
So do this:
var day = dtToday.getDate() + 4;
if(dtToday.getDay() > 2) {
day += 1;
}
So your script would be:
$(function(){
var dtToday = new Date();
var month = dtToday.getMonth() + 1;
var day = dtToday.getDate() + 4;
if(dtToday.getDay() > 2) {
day += 1;
}
var year = dtToday.getFullYear();
if(month < 10)
month = '0' + month.toString();
if(day < 10)
day = '0' + day.toString();
var maxDate = year + '-' + month + '-' + day;
$('.datepicker').attr('max', maxDate);
});

Issue removing 1 month from date - Get month 0 [duplicate]

This question already has answers here:
Adding months to a Date in JavaScript [duplicate]
(2 answers)
Closed 4 years ago.
I have this code which works fine. It gives me todays date in a specific format.
function fetchTime() {
var today = new Date();
var dd = today.getDate();
var mm = today.getMonth() + 1;
var yyyy = today.getFullYear();
if (dd < 10) {
dd = '0' + dd;
}
if (mm < 10) {
mm = '0' + mm;
}
var today = yyyy + '-' + mm + '-' + dd;
return (today);
}
I'm also trying to get today's date minus 1 month. I thought this would be simple, I just removed the +1. So I have this code:
function fetchTime() {
var today = new Date();
var dd = today.getDate();
var mm = today.getMonth();
var yyyy = today.getFullYear();
if (dd < 10) {
dd = '0' + dd;
}
if (mm < 10) {
mm = '0' + mm;
}
var today = yyyy + '-' + mm + '-' + dd;
return (today);
}
This gives me the output 2019-00-17 which should be 2018-12-17
Can anyone tell me the right way to do this? My question is specific to getting the date out in the required format, whereas most examples I have seen do not output the right format as part of the date change.

I would separate the formatting from the fetching. You could make your existing formatting function take an optional parameter that defaults to today, so you could call it like you already were for today's date.
function formatTime(date) {
var dateToFormat = date || new Date();
var dd = dateToFormat.getDate();
var mm = dateToFormat.getMonth() + 1;
var yyyy = dateToFormat.getFullYear();
if (dd < 10) {
dd = '0' + dd;
}
if (mm < 10) {
mm = '0' + mm;
}
return (yyyy + '-' + mm + '-' + dd);
}
Then you could also call it with today's date minus a month or any other date
formatTime(); //will default to today
var today = new Date();
formatTime(addMonths(today,-1)); //format last month's date
As pointed out by RobG in the comments you would need to implement an addMonths function as in Adding months to a Date in JavaScript
function addMonths(date, months) {
var d = date.getDate();
date.setMonth(date.getMonth() + +months);
if (date.getDate() != d) {
date.setDate(0);
}
return date;
}

For substracting in moment.js:
moment().subtract(1, 'months').format("DD-MM-YYYY")
Documentation:
http://momentjs.com/docs/#/manipulating/subtract/

You should subtract months by getting the current month, then subtracting the number of months you want and then updating the date variable like this.
function fetchTime() {
var today = new Date();
today.setMonth(today.getMonth() - 1);
var dd = today.getDate();
var mm = today.getMonth() + 1;
var yyyy = today.getFullYear();
if (dd < 10) {
dd = '0' + dd;
}
if (mm < 10) {
mm = '0' + mm;
}
var today = yyyy + '-' + mm + '-' + dd;
return (today);
}

Because your desired format is an ISO 8601 date, you could use JavaScript's .toISOString. You are only concerned with the first 10 characters though (not time), so you'd want to add .substring(0,10).
Date.prototype.toISODateString = function() { return this.toISOString().substring(0,10); }
Date.prototype.addMonths = function(val) { this.setMonth(this.getMonth()+val); return this;}
var date = new Date();
var todayFormatted = date.toISODateString();
console.log(todayFormatted);
var lastMonthFormatted = date.addMonths(-1).toISODateString();
console.log(lastMonthFormatted);
I've made the formatting steps a function called toISODateString() and added it to the Date prototype, which is a fancy way of saying "You can chain .toISODateString() to any Date now".
To set the date back a month, I've used .setMonth(). I also turned this into a function called addMonths.

Use the same code, but remove a month. Example:
function fetchTime() {
var today = new Date();
today.setMonth(today.getMonth() - 1);
var dd = today.getDate();
var mm = today.getMonth() + 1;
var yyyy = today.getFullYear();
if (dd < 10) {
dd = '0' + dd;
}
if (mm < 10) {
mm = '0' + mm;
}
var today = yyyy + '-' + mm + '-' + dd;
return (today);
}

Limit a date in input type date

I would like to limit the date so that the user does not have to select a date of less than one year, but I wish that it is done automatically compared to the date of the system. If anyone has ideas I'm grateful.

You can get the current date and subtract an year from the current year and set min attribute
var dtToday = new Date();
var month = dtToday.getMonth() + 1;
var day = dtToday.getDate();
var year = dtToday.getFullYear();
if(month < 10)
month = '0' + month.toString();
if(day < 10)
day = '0' + day.toString();
var minDate = (year-1) + '-' + month + '-' + day;
document.getElementById('dt').setAttribute('min', minDate);
<input type="date" id="dt">

You can specify the min and max property for the input element:
let input = document.querySelector("#test");
let nextYear = new Date(new Date().setFullYear(new Date().getFullYear() + 1));
let today = new Date();
input.max = formatDate(nextYear)
input.min = formatDate(today)
function formatDate(date) {
var d = new Date(date),
month = '' + (d.getMonth() + 1),
day = '' + d.getDate(),
year = d.getFullYear();
if (month.length < 2) month = '0' + month;
if (day.length < 2) day = '0' + day;
return [year, month, day].join('-');
}
<input type="date" id="test"/>

how to get my month and date in two digit formats

i am trying to calculate my date to 364 days using javascript, this works as i got a snippet online that functions the way i expect.
But now i have noticed that the date format is in 1 digit.
meaning for the present month instead of september to be written as "09" its wriiten as "9", i am honestly not a javascript king so am looking for help on this.
This is what i am presently trying that gives me a 1 digit date
<script type='text/javascript'>//<![CDATA[
function getdate() {
var tt = document.getElementById('inputDater').value;
var date = new Date(tt);
var newdate = new Date(date);
newdate.setDate(newdate.getDate() + 364);
var dd = newdate.getDate();
var mm = newdate.getMonth() + 1;
var y = newdate.getFullYear();
var someFormattedDate = y + '-' + mm + '-' + dd;
document.getElementById('follow').value = someFormattedDate;
}
//]]>
</script>
so onclick of my button i call my function, please can someone enlighten me thanks

Format the month into 2 digit format using the following method.
mm = ("0" + mm).slice(-2);
var someFormattedDate = y + '-' + mm + '-' + dd;
or use the following function, which is simple and you can use for date too
function pad(d) {
return (d < 10) ? '0' + d.toString() : d.toString();
}
mm = pad(mm);

you can change the line in your code from
var mm = newdate.getMonth() + 1;
to
var mm = newdate.getMonth()<9 ? '0'+(newdate.getMonth()+ 1) : newdate.getMonth()+1;

Hope this helps
function getDate() {
//var tt = document.getElementById('inputDater').value;
var date = new Date();//(tt);
var newdate = new Date(date);
newdate.setDate(newdate.getDate() + 364);
var dd = newdate.getDate();
var mm = newdate.getMonth() + 1;
var y = newdate.getFullYear();
//2 digit format
dd = dd.toString().length == 1 ? '0' + dd : dd;
mm = mm.toString().length == 1 ? '0' + mm : mm;
var someFormattedDate = y + '-' + mm + '-' + dd;
console.log(someFormattedDate);
};
getDate();