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Javascript subtract one day from "yyyy-mm-ss" string IN UTC TIMEZONE

I have a string for a UTC date var latestDate='2020-11-17' , and I'm trying to get the previous days date from this string into a new variable var subtractedDate;.
So my goal is to get subtractedDate=2020-11-16
var latestDate='2020-11-17';
//convert to iso date string
var dateStr = new Date(latestDate).toISOString();
console.log('dateStr=', dateStr);
//subtract a day
//ERROR OCCURS HERE, has trouble running // var subtractedDate = dateStr.setDate(('2020-11-17T00:00:00.000Z').getDate()-1);, something with how I have '2020-11-17T00:00:00.000Z' formatted?
var subtractedDate = dateStr.setDate(dateStr.getDate()-1);
console.log('subtractedDate = ', subtractedDate);
I am trying to use ('2020-11-17T00:00:00.000Z').getDate()-1 to subtract a day from the datetimestamp but it causes an error saying Uncaught TypeError: dateStr.getDate is not a function

We should be able to use Date.parse to get the number of milliseconds since January 1, 1970, 00:00:00 UTC, then subtract 1 days worth of milliseconds (246060*1000) to get the unix time one day earlier.
We can then use Date.toLocaleTimeString to format it.
const latestDate='2020-11-17';
// Get the number of milliseconds since 1970-1-1, then subtract 1 day (24*60*60*1000 milliseconds)
const dt = new Date(Date.parse(latestDate) - 24*60*60*1000);
// Format an ISO-8601 date in the UTC timezone
const subtractedDate = dt.toLocaleDateString('sv', { timeZone: 'UTC' });
console.log({ latestDate, subtractedDate })

Please try as follows.
dateStr.setDate(dateStr.getDate()-1);
var dateStr = new Date();
var month = dateStr.getUTCMonth() + 1; //months from 1-12
var day = dateStr.getUTCDate();
var year = dateStr.getUTCFullYear();
newdate = year + "/" + month + "/" + day;

Today's date -30 days in JavaScript

I need to get today's date -30 days but in the format of: "2016-06-08"
I have tried setDate(date.getDate() - 30); for -30 days.
I have tried date.toISOString().split('T')[0] for the format.
Both work, but somehow cannot be used together.

setDate() doesn't return a Date object, it returns the number of milliseconds since 1 January 1970 00:00:00 UTC. You need separate calls:
var date = new Date();
date.setDate(date.getDate() - 30);
var dateString = date.toISOString().split('T')[0]; // "2016-06-08"

You're saying that those two lines worked for you and your problem is combining them. Here is how you do that:
var date = new Date();
date.setDate(date.getDate() - 30);
document.getElementById("result").innerHTML = date.toISOString().split('T')[0];
<div id="result"></div>
If you really want to subtract exactly 30 days, then this code is fine, but if you want to subtract a month, then obviously this code doesn't work and it's better to use a library like moment.js as other have suggested than trying to implement it by yourself.

Please note that you would be better to use something like moment.js for this rather than reinventing the wheel. However a straight JS solution without libraries is something like:
var date = new Date();
date.setDate(date.getDate() - 30);
sets date to 30 days ago. (JS automatically accounts for leap years and rolling over months less than 30 days, and into the previous year)
now just output it like you want (gives you more control over the output). Note we are prepending a '0' so that numbers less than 10 are 0 prefixed
var dateString = date.getFullYear() + '-' + ("0" + (date.getMonth() + 1)).slice(-2) + '-' + ("0" + date.getDate()).slice(-2)

// Format date object into a YYYY-MM-DD string
const formatDate = (date) => (date.toISOString().split('T')[0]);
const currentDate = new Date();
// Values in milliseconds
const currentDateInMs = currentDate.valueOf();
const ThirtyDaysInMs = 1000 * 60 * 60 * 24 * 30;
const calculatedDate = new Date(currentDateInMs - ThirtyDaysInMs);
console.log(formatDate(currentDate));
console.log(formatDate(calculatedDate));

Today's date -30 days in this format: "YYYY-MM-DD":
var date = new Date();
date.setDate(date.getDate() - 30);
var dateString = date.toISOString().split('T')[0]; // "2021-02-05"
Today's date -30 days but get all days in this format: "YYYY-MM-DD":
var daysDate = [];
for(var i = 1; i<= 30; i++) {
var date = new Date();
date.setDate(date.getDate() - i);
daysDate.push(date.toISOString().split('T')[0]); // ["2021-02-05", "2021-02-04", ...]
}

Simply you can calculate in terms of timestamp
var date = new Date(); // Current date
console.log(date.toDateString())
var pre_date = new Date(date.getTime() - 30*24*60*60*1000);
// You will get the Date object 30 days earlier to current date.
console.log(pre_date.toDateString())
Here 30*24*60*60*1000 refers to time difference in miliseconds.

Time between two times on current date

I am trying to calculate the time between two times on the current date using JavaScript. There are other questions similar to this one, but none seem to work, and few with many upvotes that I can find.
I have the following, which fails on the line: var diff = new Date(time1 - time2);, which always gives me an invalid Date when alerted, so it is clearly failing. I cannot work out why.
The initial date is added in the format of: hh:mm:ss in an input field. I am using jQuery.
$(function(){
$('#goTime').click(function(){
var currentDate = new Date();
var dateString = (strpad(currentDate.getDate()) +'-'+ strpad(currentDate.getMonth()+1)+'-'+currentDate.getFullYear()+' '+ $('#starttime').val());
var time1 = new Date(dateString).getTime();
var time2 = new Date().getTime();
var diff = new Date(time1 - time2);
var hours = diff.getHours();
var minutes = diff.getMinutes();
var seconds = diff.getMinutes();
alert(hours + ':' + minutes + ':' + seconds);
});
});
function strpad(val){
return (!isNaN(val) && val.toString().length==1)?"0"+val:val;
}
dateString is equal to: 14-01-2013 23:00

You have the fields in dateString backwards. Swap the year and day fields...
> new Date('14-01-2013 23:00')
Invalid Date
> new Date('2013-01-14 23:00')
Mon Jan 14 2013 23:00:00 GMT-0800 (PST)

dd-MM-yyyy HH:mm is not recognized as a valid time format by new Date(). You have a few options though:
Use slashes instead of dashes: dd/MM/yyyy HH:mm date strings are correctly parsed.
Use ISO date strings: yyyy-MM-dd HH:mm are also recognized.
Build the Date object yourself.
For the second option, since you only really care about the time, you could just split the time string yourself and pass them to Date.setHours(h, m, s):
var timeParts = $('#starttime').val().split(':', 2);
var time1 = new Date();
time1.setHours(timeParts[0], timeParts[1]);

You are experiencing an invalid time in your datestring. time1 is NaN, and so diff will be. It might be better to use this:
var date = new Date();
var match = /^(\d+):(\d+):(\d+)$/.exec($('#starttime').val()); // enforcing format
if (!match)
return alert("Invalid input!"); // abort
date.setHours(parseInt(match[1], 10));
date.setMinutes(parseInt(match[2], 10));
date.setSeconds(parseInt(match[3], 10));
var diff = Date.now() - date;

If you are trying to calculate the time difference between two dates, then you do not need to create a new date object to do that.
var time1 = new Date(dateString).getTime();
var time2 = new Date().getTime();
var diff = time1 - time2;// number of milliseconds
var seconds = diff/1000;
var minutes = seconds/60;
var hours = minutes/60;
Edit: You will want to take into account broofa's answer as well to
make sure your date string is correctly formatted
The getTime function returns the number of milliseconds since Jan 1, 1970. So by subtracting the two values you are left with the number of milliseconds between each date object. If you were to pass that value into the Date constructor, the resulting date object would not be what you are expecting. see getTime

Get time difference in javascript ISO format

I have a datetime in ISO format i.e.
2012-06-26T01:00:44Z
I want to get the time difference from current time. How can I achieve this using javascript or javascript library Date.js or jquery

This will give you the difference in milliseconds, you can then format it as you want
var diff = new Date("2012-06-26T01:00:44Z") - new Date();

Try this:
var someDate = new Date("2012-06-26T01:00:44Z");
var now = new Date();
var one_day = 1000 * 60 * 60 * 24;
var diff = Math.ceil((someDate.getTime()-now .getTime())/(one_day))
alert(diff)
Example fiddle
You can obviously amend the one_day variable to get the difference in the unit you require.

I would suggest converting ISO format to something that works cross browser.
Try this,
var d = "2012-06-26T01:00:44Z";
var someDate = new Date(d.replace(/-/g,'/').replace('T',' ').replace('Z',''));
alert(someDate - new Date());
Edit:
I guess, you need pretty time
Try this awesome code
Edit 2:
You needed reverse, so try this instead
var old_date = new Date();
alert('Old date: ' + old_date.toGMTString())
var new_date = new Date(old_date.setMinutes(old_date.getMinutes() - 5));
alert('Date 5 minutes before: ' + new_date.toGMTString());
If you need timestamp,
alert(new_date.getTime());

in order to format date you can use this function to get the desire format of the date and you can easily change the position of day , month and year.
function convertFormat(inputDate)
var date = new Date(inputDate);
var day = date.getDate();
var month = date.getMonth()+1;
var year = date.getFullYear();
var fullYear = year + '/' + month + '/' + day
return fullYear;

How to add weeks to date using javascript?

Javascript definitely isn't my strongest point. I've been attempting this for a couple of hours now and seem to be getting stuck with date formatting somewhere.
I have a form where a user selected a date (dd/mm/yyyy) and then this date will be taken and 2 weeks will be added to it and then date will be copied to another form field.
My latest attempt below isn't even adding a date yet just copying the selected date in one form field to another, if I select '03/02/2012', it outputs 'Fri Mar 02 2012 00:00:00 GMT+0000 (GMT Standard Time)', so its outputting in American format as well as the full date. How to I get it to out put in the same format and add 2 weeks?
function LicenceToOccupy(acceptCompletionDate)
{
var date1 = new Date(acceptCompletionDate);
document.frmAccept.acceptLicence.value = date1;
}

You can do this :
const numWeeks = 2;
const now = new Date();
now.setDate(now.getDate() + numWeeks * 7);
or as a function
const addWeeksToDate = (dateObj,numberOfWeeks) => {
dateObj.setDate(dateObj.getDate()+ numberOfWeeks * 7);
return dateObj;
}
const numberOfWeeks = 2
console.log(addWeeksToDate(new Date(), 2).toISOString());
You can see the fiddle here.
According to the documentation in MDN
The setDate() method sets the day of the Date object relative to the beginning of the currently set month.

This might not answer the question per se, but one can find a solution with these formulas.
6.048e+8 = 1 week in milliseconds
Date.now() = Now in milliseconds
Date.now() + 6.048e+8 = 1 week from today
Date.now() + (6.048e+8 * 2) = 2 weeks from today
new Date( Date.now() + (6.048e+8 * 2) ) = Date Object for 2 weeks from today

You're assigning date1 to be a Date object which represents the string you pass it. What you're seeing in the acceptLicense value is the toString() representation of the date object (try alert(date1.toString()) to see this).
To output as you want, you'll have to use string concatenation and the various Date methods.
var formattedDate = date1.getDate() + '/' + (date1.getMonth() + 1) + '/' + date1.getFullYear();
In terms of adding 2 weeks, you should add 14 days to the current date;
date1.setDate(date.getDate() + 14);
... this will automatically handle the month increase etc.
In the end, you'll end up with;
var date1 = new Date(acceptCompletionDate);
date1.setDate(date1.getDate() + 14);
document.frmAccept.acceptLicence.value = date1.getDate() + '/' + (date1.getMonth() + 1) + '/' + date1.getFullYear();
N.B Months in JavaScript are 0-indexed (Jan = 0, Dec = 11), hence the +1 on the month.
Edit: To address your comment, you should construct date as follows instead, as the Date argument is supposed to be "A string representing an RFC2822 or ISO 8601 date." (see here).
var segments = acceptCompletionDate.split("/");
var date1 = new Date(segments[2], segments[1], segments[0]);

This should do what you're looking for.
function LicenceToOccupy(acceptCompletionDate)
{
var date1 = new Date(acceptCompletionDate);
date1.setDate(date1.getDate() + 14);
document.frmAccept.acceptLicence.value = date1.getDate() + '/' + (date1.getMonth() + 1) + '/' + date1.getFullYear();
}

To parse the specific dd/mm/yyyy format and increment days with 14 , you can do something like split the parts, and create the date object with y/m/d given specfically. (incrementing the days right away) Providing the separator is always -, the following should work:
function LicenceToOccupy(acceptCompletionDate)
{
var parts = acceptCompletionDate.split("/");
var date1 = new Date(parts[2], (parts[1] - 1), parseInt(parts[0]) + 14); //month 0 based, day: parse to int and increment 14 (2 weeks)
document.frmAccept.acceptLicence.value = date1.toLocaleDateString(); //if the d/m/y format is the local string, otherwise some cusom formatting needs to be done
}

date1.toLocaleDateString()
Thiswill return you date1 as a String in the client convention
To create a new date date2 with 2 weeks more (2weeks = 27246060 seconds):
var date2 = new Date(date1 + 60*60*24*7*2);