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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.

JavaScript, get date of the next day [duplicate]

This question already has answers here:
Incrementing a date in JavaScript
(19 answers)
Closed 8 years ago.
I have the following script which returns the next day:
function today(i)
{
var today = new Date();
var dd = today.getDate()+1;
var mm = today.getMonth()+1;
var yyyy = today.getFullYear();
today = dd+'/'+mm+'/'+yyyy;
return today;
}
By using this:
today.getDate()+1;
I am getting the next day of the month (for example today would get 16).
My problem is that this could be on the last day of the month, and therefore end up returning 32/4/2014
Is there a way I can get the guaranteed correct date for the next day?

You can use:
var tomorrow = new Date();
tomorrow.setDate(tomorrow.getDate()+1);
For example, since there are 30 days in April, the following code will output May 1:
var day = new Date('Apr 30, 2000');
console.log(day); // Apr 30 2000
var nextDay = new Date(day);
nextDay.setDate(day.getDate() + 1);
console.log(nextDay); // May 01 2000
See fiddle.

Copy-pasted from here:
Incrementing a date in JavaScript
Three options for you:
Using just JavaScript's Date object (no libraries):
var today = new Date();
var tomorrow = new Date(today.getTime() + (24 * 60 * 60 * 1000));
One-liner
const tomorrow = new Date(new Date().getTime() + (24 * 60 * 60 * 1000));
Or if you don't mind changing the date in place (rather than creating
a new date):
var dt = new Date();
dt.setTime(dt.getTime() + (24 * 60 * 60 * 1000));
Edit: See also Jigar's answer and David's comment below: var tomorrow
= new Date(); tomorrow.setDate(tomorrow.getDate() + 1);
Using MomentJS:
var today = moment();
var tomorrow = moment(today).add(1, 'days');
(Beware that add modifies the instance you call it on, rather than
returning a new instance, so today.add(1, 'days') would modify today.
That's why we start with a cloning op on var tomorrow = ....)
Using DateJS, but it hasn't been updated in a long time:
var today = new Date(); // Or Date.today()
var tomorrow = today.add(1).day();

Using Date object guarantees that. For eg if you try to create April 31st :
new Date(2014,3,31) // Thu May 01 2014 00:00:00
Please note that it's zero indexed, so Jan. is 0, Feb. is 1 etc.

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

Convert Returned String (YYYYMMDD) to Date

I have a string that contains 8 digits that represent a date. For example:
20120515
I'd like to compare it with today's date, created in this manner:
var currentDate = new Date();
How can I convert the "8 digit date string" to a suitable date format in order to compare it to currentDate?

Use the substring method and substring off 4 elements and assign it to your new date for the year. Then substring off two elements at a time and store the month and date accordingly.
var dateString = "20120515";
var year = dateString.substring(0,4);
var month = dateString.substring(4,6);
var day = dateString.substring(6,8);
var date = new Date(year, month-1, day);
var currentDate = new Date();
Now you can compare the two dates with the normal operators.

If you want a small date library you can use moment.js.
var a = moment("20120515", "YYYYMMDD");
// then use any of moment's manipulation or display functionality
a.format("MMM Do YYYY"); // May 15th 2012
a.fromNow(); // 14 hours ago
a.calendar(); // Today at 12:00 AM

To correctly handle the local time zone, it must explicitly summed to the calculated time
function dateStringToDate(dateString) {
try {
var year = dateString.substring(0, 4);
var month = dateString.substring(4, 6);
var day = dateString.substring(6, 8);
var date = new Date(year, month - 1, day);
const offset = date.getTimezoneOffset()
date = new Date(date.getTime() - (offset * 60 * 1000));
return date;
} catch (error) {
return null;
}
}
function dateStringToDate(dateString) {
try {
var year = dateString.substring(0, 4);
var month = dateString.substring(4, 6);
var day = dateString.substring(6, 8);
var date = new Date(year, month - 1, day);
const offset = date.getTimezoneOffset()
date = new Date(date.getTime() - (offset * 60 * 1000));
return date;
} catch (error) {
return null;
}
}
console.log(dateStringToDate("20211212"))
console.log(dateStringToDate("20211213"))
console.log(dateStringToDate("20211214"))

...some other "one-liner" ways to accomplish this:
(They take a value like dts='20020704'; and return date object [dt].)
var dt=new Date(dts.slice(0,4), (dts[4]+dts[5])-1, dts[6]+dts[7]);
...or...
var m=dts.match(/(....)(..)(..)/), dt=new Date(m[1],m[2]-1,m[3]);
...or...
var m=dts.match(/.{1,2}/g), dt=new Date(m[0]+m[1],m[2]-1,m[3]);
The last one's shortest, but the first is probably most efficient, since it doesn't use regex (but that's irrelevant, unless you're processing LOTS of data using this). I like the middle one best since it's easy to see what's happening.

Using answers from prompt for a calculation

Total newbie at JavaScript.
I would like to calculate how many days one has been alive by asking the user their date of birth via prompts/alerts, then obviously subtracting their date of birth from today's date.
I've made a bit of a start...
var month=prompt("Please enter month of birth"," ");
var day=prompt("Please enter day of birth"," ");
var year=prompt("Please enter your year of birth"," ");
var curdate = this is the bit i need help with
var birth = this is the bit i need help with
var milliDay = 1000 * 60 * 60 * 24; // a day in milliseconds;
var ageInDays = (curdate - birth) / milliDay;
document.write("You have been alive for: " + ageInDays);
Any advice or help would be much appreciated.

You need to use the Date object (MDN). They can be created from a month, a day, and a year, and added/subtracted.
Typically :
var curDate = new Date();
var birth = new Date(year, month, day);
var ageInDays = (curdate.getTime() - birth.getTime()) / milliDay;
Be aware of the fact that months starts at 0, e.g. January is 0.

var curDate = new Date();
gives you the current date.
var birthdate = new Date(year, month-1, day);
gives you a Date from the separate variables. NB the month is zero-based.

end = Date.now(); // Get current time in milliseconds from 1 Jan 1970
var date = 20; //Date you got from the user
var month = 8-1; // Month, subtracted by one because month starts from 0 according to JS
var year = 1996; // Year
//Set date to the old time
obj = new Date();
obj.setDate(date);
obj.setMonth(month);
obj.setYear(year);
obj = obj.getTime(); //Get old time in milliseconds from Jan 1 1970
document.write((end-obj)/(1000*60*60*24));
Simply subtract current time from Jan 1 1970 in milliseconds from their birthdate's time from Jan 1 1970 in milliseconds. Then convert it to days. Look at MDN's Docs for more info.
See JSFiddle for a working example. Try entering yesterday's date. It should show 1 day.

Read some of this: http://www.w3schools.com/js/js_obj_date.asp