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Convert string to date and alter that date

Good day, I am generating 3 dates from a string, I hope the output was:
billing date: 2020/01/11
cutoff start: 2019/11/11
cuttof end: 2019/12/10
but I get the following:
billing date: 2020/11/10
cutoff start: 2019/11/10
cuttof end: 2019/12/10
I would like to know how javascript works with variables or what is the problem since everything is altered
var month = "Jan-20"
var coverage_month_obj = moment(month, 'MMM-YY').toDate();
var billing_date = new Date(coverage_month_obj.setDate(coverage_month_obj.getDate() + 10))
var cutoff_end = new Date(billing_date.setMonth(billing_date.getMonth() - 1))
cutoff_end = new Date(billing_date.setDate(billing_date.getDate() - 1))
var cutoff_start = new Date(billing_date.setMonth(billing_date.getMonth() - 1))

I would like to know how javascript works with variables or what is the problem since everything is altered
Put simply, calling setXXX on a javascript date variable updates that variable in place. ie, it is what we would call "mutable". You might have assumed dates were immutable and did not change in place.
To answer on a better way to achieve your goal, I'd suggest using the other functionality of momentjs to calculate your 3 dates from the given input string.
var month = "Jan-20"
var coverage_month = moment(month, 'MMM-YY');
//Jan-20 I need to convert it into date format and that the day is 11 (2020/01/11) cutoff start, are two months less from that date (2020/11/11) and cutoff end is one month less from Jan-20, but ends on day 10 (2020/12/10)
var billing_date = coverage_month.clone().add(10, 'days');
var cutoff_start = billing_date.clone().subtract(2, 'months');
var cutoff_end = billing_date.clone().subtract(1,'months').subtract(1,'day')
console.log("billing_date",billing_date);
console.log('cutoff_start',cutoff_start);
console.log('cutoff_end',cutoff_end);
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.24.0/moment.min.js"></script>

Moment check next month

I'd like to check the date if it's next month from the current.
Some test cases here
2020.1.1 2019.12.30 // true
2019.11.30 2019.10.10 // true
2019.12.11 2019.12.1 // false
So as you can see, I'd like to check if the date is next month from now.
Hope to get the brilliant idea!
Best
Kinji

You could try this approach, we have to handle the case when the dates are in the same year and also for December and January of the next year.
A simple algorithm can be created, whereby we multiply the year by 12, add the month and get the difference of this score between the two dates. As long as the value of this is 1 or -1 we have months that are adjacent. I'm presuming you only need to check if the first parameter is the next month to the second parameter, so we check for a difference of 1.
function isNextMonth(timeStamp1, timeStamp2, format = "YYYY.MM.DD") {
let dt1 = moment(timeStamp1, format);
let dt2 = moment(timeStamp2, format);
return ((dt1.year()*12 + dt1.month() - dt2.year()*12 - dt2.month()) === 1);
}
let inputs = [['2020.1.1','2019.12.30'],['2019.11.30', '2019.10.10'], ['2019.12.11','2019.12.1'], ['2020.12.1','2021.1.1'], ['2020.6.1','2020.5.1'], ['2021.1.1','2019.12.1'], ['2019.3.1', '2019.4.1']];
for(let [inputa, inputb] of inputs) {
console.log(`isNextMonth(${inputa}, ${inputb}): ${isNextMonth(inputa, inputb)}`);
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.24.0/moment.min.js"></script>

A very simple solution would be to use Moment’s isSame. This only checks the month. If you also want to make sure that the year matches you would have to make sure that nextMonth.getYear() matches dateToCheck.getYear()
var dateToCheck = new Date(2019, 11, 30)
var nextMonth = new Date()
nextMonth.setMonth(dateToCheck.getMonth() + 1)
moment(dateToCheck).isSame(nextMonth, 'month');

JavaScript - convert to string using array index

I got this code from a book and I don't understand how this statement "mm=months[mm]" convert to month name string:
function init(){
var panel = document.getElementById("panel");
var days=["sun","mon","tue","wed","thur","fri","sat"];
var months=["jan","feb","mar","apr","may","jun",
"jul","aug","sep","oct","nov","dec"];
var now = new Date();
var yy = now.getFullYear();
var mm = now.getMonth();
var dd = now.getDate();
var dy = now.getDay();
mm=months[mm]; //convert to month name string
dy=days[dy]; //convert to month name string
var str = dy+","+mm","+dd+","+yy;
panel.innerHTML+="us date string: "+str;
str = dy+","+dd+" "+mm+","+yy;
panel.innerHTML+="<br>uk date string: "+str;
}
window.onload=init();
My question is this what exactly does the mm=months[mm] and dy=days[dy] (convert to month or day name string) do. I don't understand how this statement "mm=months[mm]" convert to month name string, when months is an array. Is this just some build in function of an array?

mm is a number at the beginning of the statement mm=months[mm], so therefore it returns the mmth value in the months array which is a string. In JavaScript variables are not strictly typed so it then inserts this string into the variable mm

mm is a number from 0-11 that represents the month specific to the date object you retrieved it from. months is an array containing the names of all the months. If you say mm = months[mm], you're saying that mm equals the string at position mm in the months array. It's confusing, because they're using the same variable name. It might have been better to do something like this:
var mm = now.getMonth();
alert(mm); // will alert 6
var monthName = months[mm]; // will alert July, which is position 7 in the months array
The reason mm is six is because the months for some reason are 0-indexed, meaning they start at 0 and go to 11; January would be 0, and December 11.

days is an array. Given index dy=0, days[dy] returns "sun", which is the 0th value in the array. Similarly, given index dy=1, days[dy] returns "mon", which is the 1st value in the array. And so on.
Same concept for months.
But the correct answer should be "use some built-in function in Javascript or jQuery". See this post for more details. Rolling your own datetime conversion functions is almost certainly going to come back and bite you in the long run.

Assume the date is 08 and month is july .
var yy = now.getFullYear();
var mm = now.getMonth(); // month is 7 now
var dd = now.getDate(); // dd is 8 now
var dy = now.getDay();
Now,
mm=months[mm];
means,
mm = months[7];
// in months array, at seventh index we have july,
so mm is july.
No its not converting but using value from array based on index.

mm=month[mm] take the value at the index mm (returned by now.getMonth()) in the months array and puts it in the mm variable. That id the same thing for dy=days[dy].
I recognize that the code is badly written. mm2=month[mm] and use mm2 after (instead of the "new" mm) should be written instead.

the closest Sunday before given date with JavaScript

I need to know the date for last Sunday for given date in php & javascript
Let's have a function give_me_last_Sunday
give_me_last_Sunday('20110517') is 20110515
give_me_last_Sunday('20110604') is 20110529
The full backup is done on Sundays = weekly. If I want to restore daily backup I need full (weekly) and daily backup. I need to copy backup files before restoring to temp directory so I restoring daily backup I need to know what weekly backup file I need to copy along the daily file.
My thought was to get Julian representation (or something similar) for the given date and then subtract 1 and check if it is Sunday ... Not sure if this is the best idea and how to convert given date into something I can subtract.

Based on Thomas' effort, and provided the input string is exactly the format you specified, then:
function lastSunday(d) {
var d = d.replace(/(^\d{4})(\d{2})(\d{2}$)/,'$1/$2/$3');
d = new Date(d);
d.setDate(d.getDate() - d.getDay());
return d;
}
Edit
If I were to write that now, I'd not depend on the Date object parsing the string but do it myself:
function lastSunday(s) {
var d = new Date(s.substring(0,4), s.substring(4,6) - 1, s.substring(6));
d.setDate(d.getDate() - d.getDay());
return d;
}
While the format yyyy/mm/dd is parsed correctly by all browsers I've tested, I think it's more robust to stick to basic methods. Particularly when they are likely more efficient.

Ok so this is for JavaScript only. You have an input that you need to extract the month, date, and year from. The following is just partly an answer then on how to get the date:
<script type="text/javascript">
var myDate=new Date();
myDate.setFullYear(2011,4,16)
var a = myDate.getDate();
var t = myDate.getDay();
var r = a - t;
document.write("The date last Sunday was " + r);
</script>
So the setFullYear function sets the myDate to the date specified where the first four digits is the year, the next are is the month (0= Jan, 1= Feb.,...). The last one is the actually date. Then the above code gives you the date of the Sunday before that. I am guessing that you can add more code to get the month (use getMonth() method). Here are a few links that might be helpful
http://www.w3schools.com/js/js_obj_date.asp
http://www.w3schools.com/jsref/jsref_setFullYear.asp
http://www.w3schools.com/jsref/jsref_getMonth.asp
(You can probably find the other functions that you need)
I hope this helps a bit even though it is not a complete answer.

Yup and strtotime has been ported to JS for eg http://phpjs.org/functions/strtotime:554 here.

final code (big thanks to #Thomas & #Rob)
function lastSunday(d) {
var d = d.replace(/(^\d{4})(\d{2})(\d{2}$)/,'$1/$2/$3');
d = new Date(d);
d.setDate(d.getDate() - d.getDay());
year = d.getFullYear()+'';
month = d.getMonth()+1+'';
day = d.getDate()+'';
if ( month.length == 1 ) month = "0" + month; // Add leading zeros to month and date if required
if ( day.length == 1 ) day = "0" + day;
return year+month+day;
}

Subtracting date from an input field

I would like to have an input field with a button next to it.
On the input field I will enter a date like this:
2011-07-08
And when I hit the button it should read the time that has been entered on the input field and subtract it with 3 months and one day.
Is this possible?
Thanks in advance

Yes. First you read the date and you convert to a date object
var dateString = document.getElementById('id of your field').value,
date = new Date(dateString);
then you subtract 91 days and output the result
date.setDate(date.getDate() - 91);
alert(date.toString());
Here I assume for simplicity that you actually want 91 days and not 3 months and one day. If you want three months and one day you will do
date.setMonth(date.getMonth() - 3);
date.setDate(date.getDate() - 1);
alert(date.toString());
The Date object will take care of overflows, leap years and everything.
If you want to write it to same field, taking care of zeroes, you can do
function assureTwoDigits(number) {
if (number > 9) {
return '-' + number;
}
else {
return '-0' + number;
}
}
and change the last line to
document.getElementById('id of your field').value = date.getFullYear() + assureToDigits(date.getMonth()) + assureTwoDigits(date.getDate());

You can use Date objects (see here):
extract year, moth and day from the string (using a regular expression or splitting by '-')
buid a new Date object with that data
subtract the date interval
build the string back

The simplest way would be to split it into an array, then use a couple of if/else statements:
var date = (whatever you're pulling the date in as).split('-');
if (date[1] > 3)
date[1] = date[1] - 3;
else
date[0] = date[0] - 1;
var dateOverflow = date[1]-3;
date[1] = 12 - dateOverflow;
And then the same for the days.

Yes, it's possible and it's the most clean if you can do it without some arcane regex magic. Start by converting the date to a Date object:
// this will get you a date object from the string:
var myDate = new Date("2011-07-08");
// subtract 3 months and 1 day
myDate.setMonth(myDate.getMonth()-3);
myDate.setDay(myDate.getMonth(), myDate.getDay()-1);
// And now you have the day and it will be correct according to the number of days in a month etc
alert(myDate);