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Checking if date is greater than today

Trying out a Jquery to confirm if date selected is equal to today or greater than.
If i select today, it return it as the day selected is less than today. Selecting previous day works well but selecting today returns less than. Any tip.
var firstRepaymentDate = new Date($('#First_Repayment_Date').val());
var today = new Date();
if (firstRepaymentDate.getTime() < today.getTime()) {
alert('The First Repayment Date Can only Be Today Or Future Date');
return false;
}

Don't forget that new Date() will include the current time as well. You'll need to remove that time component with today.setHours(0,0,0,0) for the comparison to be correct.
Also, setHours() returns the underlying value like getTime() so you can do
var firstRepaymentDate = new Date($('#First_Repayment_Date').val());
var today = new Date();
if (firstRepaymentDate.getTime() < today.setHours(0,0,0,0)) {
alert('The First Repayment Date Can only Be Today Or Future Date');
return false;
}
In response to the comment about adding 20 days:
This is a little more detailed but is fairly easy.
var today = new Date();
var plus20Days = new Date(today.setDate(today.getDate() + 20));
again you can then use setHours() to reset the time component.

new Date() considers time too, not only the date. I think the easiest way to achieve this is to compare years, months and days by using respectively getFullYear() , getMonth() , getDate().
Check all the methods that manipulate js Date here
https://developer.mozilla.org/it/docs/Web/JavaScript/Reference/Global_Objects/Date

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.

Simple javascript date math... not really

I am trying to create a simple script that gives me the next recycling date based on a biweekly schedule starting on Wed Jul 6, 2011. So I've created this simple function...
function getNextDate(startDate) {
if (today <= startDate) {
return startDate;
}
// calculate the day since the start date.
var totalDays = Math.ceil((today.getTime()-startDate.getTime())/(one_day));
// check to see if this day falls on a recycle day
var bumpDays = totalDays%14; // mod 14 -- pickup up every 14 days...
// pickup is today
if (bumpDays == 0) {
return today;
}
// return the closest day which is in 14 days, less the # of days since the last
// pick up..
var ms = today.getTime() + ((14- bumpDays) * one_day);
return new Date(ms);
}
and can call it like...
var today=new Date();
var one_day=1000*60*60*24; // one day in milliseconds
var nextDate = getNextDate(new Date(2011,06,06));
so far so good... but when I project "today" to 10/27/2011, I get Tuesday 11/8/2011 as the next date instead of Wednesday 11/9/2011... In fact every day from now thru 10/26/2011 projects the correct pick-up... and every date from 10/27/2011 thru 2/28/2012 projects the Tuesday and not the Wednesday. And then every date from 2/29/2012 (leap year) thru 10/24/2012 (hmmm October again) projects the Wednesday correctly. What am I missing? Any help would be greatly appreciated..
V

The easiest way to do this is update the Date object using setDate. As the comments for this answer indicate this isn't officially part of the spec, but it is supported on all major browsers.
You should NEVER update a different Date object than the one you did the original getDate call on.
Sample implementation:
var incrementDate = function (date, amount) {
var tmpDate = new Date(date);
tmpDate.setDate(tmpDate.getDate() + amount)
return tmpDate;
};
If you're trying to increment a date, please use this function. It will accept both positive and negative values. It also guarantees that the used date objects isn't changed. This should prevent any error which can occur if you don't expect the update to change the value of the object.
Incorrect usage:
var startDate = new Date('2013-11-01T11:00:00');
var a = new Date();
a.setDate(startDate.getDate() + 14)
This will update the "date" value for startDate with 14 days based on the value of a. Because the value of a is not the same is the previously defined startDate it's possible to get a wrong value.

Expanding on Exellian's answer, if you want to calculate any period in the future (in my case, for the next pay date), you can do a simple loop:
var today = new Date();
var basePayDate = new Date(2012, 9, 23, 0, 0, 0, 0);
while (basePayDate < today) {
basePayDate.setDate(basePayDate.getDate()+14);
}
var nextPayDate = new Date(basePayDate.getTime());
basePayDate.setDate(nextPayDate.getDate()-14);
document.writeln("<p>Previous pay Date: " + basePayDate.toString());
document.writeln("<p>Current Date: " + today.toString());
document.writeln("<p>Next pay Date: " + nextPayDate.toString());
This won't hit odd problems, assuming the core date services work as expected. I have to admit, I didn't test it out to many years into the future...

Note: I had a similar issue; I wanted to create an array of dates on a weekly basis, ie., start date 10/23/2011 and go for 12 weeks. My code was more or less this:
var myDate = new Date(Date.parse(document.eventForm.startDate.value));
var toDate = new Date(myDate);
var week = 60 * 60 * 24 * 7 * 1000;
var milliseconds = toDate.getTime();
dateArray[0] = myDate.format('m/d/Y');
for (var count = 1; count < numberOccurrences; count++) {
milliseconds += week;
toDate.setTime(milliseconds);
dateArray[count] = toDate.format('m/d/Y');
}
Because I didn't specify the time and I live in the US, my default time was midnight, so when I crossed the daylight savings time border, I moved into the previous day. Yuck. I resolved it by setting my time of day to noon before I did my week calculation.

Get a date object (six months prior) from another date object

How can I create a date object which is less than n number of months from another date object? I am looking for something like DateAdd().
Example:
var objCurrentDate = new Date();
Now using objCurrentDate, how can I create a Date object having a date which is six months older than today's date / objCurrentDate?

You can implement very easily an "addMonths" function:
function addMonths(date, months) {
date.setMonth(date.getMonth() + months);
return date;
}
addMonths(new Date(), -6); // six months before now
// Thu Apr 30 2009 01:22:46 GMT-0600
addMonths(new Date(), -12); // a year before now
// Thu Oct 30 2008 01:20:22 GMT-0600
EDIT: As reported by #Brien, there were several problems with the above approach. It wasn't handling correctly the dates where, for example, the original day in the input date is higher than the number of days in the target month.
Another thing I disliked is that the function was mutating the input Date object.
Here's a better implementation handling the edge cases of the end of months and this one doesn't cause any side-effects in the input date supplied:
const getDaysInMonth = (year, month) => new Date(year, month, 0).getDate()
const addMonths = (input, months) => {
const date = new Date(input)
date.setDate(1)
date.setMonth(date.getMonth() + months)
date.setDate(Math.min(input.getDate(), getDaysInMonth(date.getFullYear(), date.getMonth()+1)))
return date
}
console.log(addMonths(new Date('2020-01-31T00:00:00'), -6))
// "2019-07-31T06:00:00.000Z"
console.log(addMonths(new Date('2020-01-31T00:00:00'), 1))
// "2020-02-29T06:00:00.000Z"
console.log(addMonths(new Date('2020-05-31T00:00:00'), -6))
// "2019-11-30T06:00:00.000Z"
console.log(addMonths(new Date('2020-02-29T00:00:00'), -12))
// "2019-02-28T06:00:00.000Z"

Create date object and pass the value of n, where n is number(add/sub) of month.
var dateObj = new Date();
var requiredDate= dateObj.setMonth(dateObj.getMonth() - n);

var oldDate:Date = new Date();
/*
Check and adjust the date -
At the least, make sure that the getDate() returns a
valid date for the calculated month and year.
If it's not valid, change the date as per your needs.
You might want to reset it to 1st day of the month/last day of the month
or change the month and set it to 1st day of next month or whatever.
*/
if(oldDate.getMonth() < n)
oldDate.setFullYear(oldDate.getFullYear() - 1);
oldDate.setMonth((oldDate.getMonth() + n) % 12);

You have to be careful because dates have a lot of edge cases. For example, merely changing the month back by 6 doesn't account for the differing number of days in each month. For example, if you run a function like:
function addMonths(date, months) {
date.setMonth((date.getMonth() + months) % 12);
return date;
}
addMonths(new Date(2020, 7, 31), -6); //months are 0 based so 7 = August
The resulting date to return would be February 31st, 2020. You need to account for differences in the number of days in a month. Other answers have suggested this in various ways, by moving it to the first of the month, or the last of the month, or the first of the next month, etc. Another way to handle it is to keep the date if it is valid, or to move it to the end of the month if it overflows the month's regular dates. You could write this like:
function addMonths(date, months) {
var month = (date.getMonth() + months) % 12;
//create a new Date object that gets the last day of the desired month
var last = new Date(date.getFullYear(), month + 1, 0);
//compare dates and set appropriately
if (date.getDate() <= last.getDate()) {
date.setMonth(month);
}
else {
date.setMonth(month, last.getDate());
}
return date;
}
This at least ensures that the selected day won't "overflow" the month that it is being moved to. Finding the last day of the month with the datePart = 0 method is documented here.
This function still leaves a lot to be desired, as it doesn't add years and you can't subtract more than a year (or you will run into a new issue with negatives being involved). However, fixing those and the other issues you may run into (namely timezones) will be left as an exercise for the reader.

Find first day of previous month in javascript

Given a date object, how to get its previous month's first day in javascript

function firstDayInPreviousMonth(yourDate) {
var d = new Date(yourDate);
d.setDate(1);
d.setMonth(d.getMonth() - 1);
return d;
}
EDIT: Alright... I've definitely learned something here. I think that this is the simplest solution that covers all cases (and yes, it does work for January):
function firstDayInPreviousMonth(yourDate) {
return new Date(yourDate.getFullYear(), yourDate.getMonth() - 1, 1);
}

The following should work:
now = new Date();
if (now.getMonth() == 0) {
current = new Date(now.getFullYear() - 1, 11, 1);
} else {
current = new Date(now.getFullYear(), now.getMonth() - 1, 1);
}
keeping in mind that months are zero-based so December is 11 rather than 12.
But, as others have pointed out, the month wraps, even as part of the atomic constructor, so the following is also possible:
now = new Date();
firstDayPrevMonth = new Date(now.getFullYear(), now.getMonth() - 1, 1);

I like this solution. It might not be the briefest, but it highlights some functions of the setDate() method on Date() objects that not everybody will be familiar with:
function firstDayPreviousMonth(originalDate) {
var d = new Date(originalDate);
d.setDate(0); // set to last day of previous month
d.setDate(1); // set to the first day of that month
return d;
}
It makes use of the fact that .setDate(0) will change the date to point to the last day of the previous month, while .setDate(1) will change it (further) to point to the first day of that month. It lets the core Javascript libs do the heavy lifting.
You can see a working Plunk here.

It will help to get the previous month first and last date.
function getLastMonth(){
var now = new Date();
var lastday = new Date(now.getFullYear(), now.getMonth(), 0);
var firstday = new Date(lastday.getFullYear(), lastday.getMonth(), 1);
var lastMonth = firstday.getDate()+'/'+(firstday.getMonth()+1)+'/'+firstday.getFullYear()+' - '+lastday.getDate()+'/'+(firstday.getMonth()+1)+'/'+lastday.getFullYear();
return lastMonth;
}

Why reinventing the wheel?
Use moment.js or one of the alternatives (Luxon, Day.js, etc.):
moment().subtract(1, "months").startOf("months").toDate().

I've needed to use the begging of the month in a BootStrap Min month and didn't want to write it all out.
(() => new Date(new Date().getFullYear(), new Date().getMonth(),1, 0))()