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how to get first and last date of the current year javascript

I would like to know how to get the first and last date of the current year. Example in this format: 2020-01-01 and 2020-12-31. Thanks!
I have this initial code that gets the current date today. This might help.
var today = new Date();
var date = today.getFullYear() + '-' + ('0' + (today.getMonth()+1)).slice(-2) + '-' + ('0' + today.getDate()).slice(-2);

You can get the first date of the current year as follows.
var d = new Date(new Date().getFullYear(), 0, 1);
You can get the last date of the current year as follows.
var d = new Date(new Date().getFullYear(), 11, 31);

This is how you can calculate the current date:
var currentDate = new Date();
You can instantiate Date using year, month and day, but keep in mind that month is indexed from 0:
var theFirst = new Date(currentDate.getFullYear(), 0, 1);
var theLast = new Date(currentDate.getFullYear(), 11, 31);

This seems a lot like homework, so I'm not going to give you a code block to copy and paste.
You seem to already know how to get the current year, and the year always goes from 01-01 to 12-31. So do what you already have, but instead of today's date, use 01-01 and 12-31

Momentjs - Week Numbers When April Is Week One

Using this function in momentjs I can find the week number of the year:
var dt = new Date();
var weekNumber = moment(dt).week();
Can anyone tell me how to set the first week in April as week one, and therefore for week 52 to be the last week in March.
In the documentation I can only see how to adjust the first day of the year (ie Sunday or Monday). I need to do both. Saturday will actually be day one.
Help much appreciated.

You will have to add a custom function,
Sample
function getCustomWeekNumber(weekNo) {
var baseWeek = moment("01/04/", "DD/MM/").week() - 1; // 13
var lastWeek = moment("31/12/", "DD/MM/").week() //53;
return weekNo > baseWeek ? weekNo - baseWeek : (lastWeek - baseWeek) + weekNo;
}
var d = moment().week();
console.log(getCustomWeekNumber(d))
d = moment("01/04/2016", "DD/MM/YYYY").week();
console.log(getCustomWeekNumber(d))
d = moment("24/03/2016", "DD/MM/YYYY").week();
console.log(getCustomWeekNumber(d))
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.15.0/moment.min.js"></script>

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);

Calculate the Date based on Current Date and No of Days using Javascript/Jquery

I need a help..
I have a Current Date and No of days column.
When i enter number of days,i should add current date plus no of days entered.
For example,
todays date 5th jan + 20(no of days) = 25th Jan 2011 in another column.
Kindly help me.
Thanks in Advance.

Date.js is fantastic for this.
Date.today().add(5).days();

As you are learning JavaScript you may find the w3schools site useful for simple examples of objects and functions that are exposed and how they may be used.
http://www.w3schools.com/jsref/jsref_obj_date.asp
You can calculate the date as follows:
var d = new Date(); // Gets current date
var day = 86400000; // # milliseconds in a day
var numberOfDays = 20;
d.setTime(d.getTime() + (day*numberOfDays)); // Add the number of days in milliseconds
You can then use one of the various methods of displaying the date:
alert(d.toUTCString());

You could do something like
Date.today().add(X).days();
Where X is the number of days the user has entered.

You can add dates like this in js:
var someDate = new Date();
var numberOfDaysToAdd = 6;
someDate.setDate(someDate.getDate() + numberOfDaysToAdd);
var month = someDate.getMonth() + 1; //Add 1 because January is set to 0 and Dec is 11
var day = someDate.getDate();
var year = someDate.getFullYear();
document.write(month + "/" + day + "/" + year);
See this p.cambell's answer here: How to add number of days to today's date?

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