Develop Reference

Javascript Subtract/Add 1 month from date [duplicate]

This question already has answers here:
JavaScript function to add X months to a date
(24 answers)
Adding months to a Date in JavaScript [duplicate]
(2 answers)
Closed 2 years ago.
I'm trying to find a method that reliably subtracts 1 month from a javascript date object.
I have this code:
var shippedDate = new Date('12/31/2020');
var tempDate = new Date(shippedDate.setMonth(shippedDate.getMonth() - 1)); //subtract 1 month
alert(tempDate);
The value in tempDate after this code runs is 12/1/2020 when it should actually be 11/30/2020.
I checked my math with this online date calculator: https://www.timeanddate.com/date/dateadded.html?m1=12&d1=31&y1=2020&type=sub&ay=&am=1&aw=&ad=&rec=
Thanks.

December has 31 days so when you subtract 1 month, you get 31 November which doesn't exist, so it rolls over to 1 December.
You can test the date (day in month) to see if it's the same, and if not, set the date to 0 so it goes to the last day of the previous month.
Also, setDate modifies the Date object so no need to create a new one:
function subtractMonth(date, months) {
let d = date.getDate();
date.setMonth(date.getMonth() - months);
if (date.getDate() != d) {
date.setDate(0);
}
return date;
}
let d = new Date(2020, 11, 31); // 31 Dec 2020
console.log(subtractMonth(d, 1).toString()); // 30 Nov 2020
This has side effects so that sequentially subtracting 2 months may give a different result to subtracting 2 months in one go.
Also in regard to new Date('12/31/2020'), see see Why does Date.parse give incorrect results?
PS
I answered this before I remembered that there were plenty of questions about adding months that also cover subtracting. So I marked this question as a duplicate and rather than delete this answer, left it for posterity.
If you wish to vote for an answer, please go to one of the duplicates and vote for an answer there. :-)

On my own experience, I may qualify all around Date calculation in javascript as completely unbearable pain.
Avoid as possible own crafted function to any Date manipulation. There are too many traits to lose mind at all. Timezones, wrong clocks, timezone on your own host vs. timezone on server, unexpected toString conversion according to local host timezone/clock.
If you rally need to make some dates calculation use battle tested library, like date-fns, moment.js, etc.
By the way your example almost correct, you just have chosen not suitable time to try to test it. The only one that I see problematic it's using setMonth that mutate original shippedDate.

Date to epoch and vice versa - JavaScript

I need to convert date to Java epoch and then read it and convert back. Not sure what I'm doing wrong here?
var date = new Date('1/3/2013');
var timeStamp = date.getTime();
console.log(timeStamp);
var revertDate = new Date(timeStamp);
console.log(revertDate.getDate()+'/'+revertDate.getMonth()+'/'+revertDate.getFullYear());
The output is 3/0/2013 instad 1/3/2013?
fiddle link

You've got two problems here:
The Date constructor is assuming M/d/yyyy format - whereas you're logging d/M/yyyy format. Personally I'd suggest using an ISO-8601 format if at all possible: yyyy-MM-dd
You're not taking into account the fact that getMonth() returns a 0-based value
For the formatting side, you'd be better off using toISOString or something similar, rather than doing the formatting yourself.
(Note that looking at the documentation for the Date constructor it's not clear that the code you've got should work at all, as it's neither an RFC822 nor ISO-8601 format.)
Neither of the problems are to do with converting between Date and a numeric value. If you change your logging, you'll see that clearly:
var date = new Date('1/3/2013');
var timeStamp = date.getTime();
console.log(date);
var revertDate = new Date(timeStamp);
console.log(revertDate);

var date = new Date('1/3/2013');
The Date constructor is parsing this given string this way:
Month / Day / Year
So, in this case, Month is 1, Day is 3 and Year is 2013. What's going on there? Well that's quite simple. This Gregorian representation of a date(which is specifically Day / Month / Year ) isn't the one used by the Date constructor, so it will parse the 1(the month) as January, the 3 as the third day of the month(the third of Jan) and the year correctly, the 2013. Now, due to its 0-based indexing, the constructed Date object will return a month which is n-1 among the one provided. That's why you're getting 3/0/2013. It is the third day(3) of the month 0(which is January) of 2013. If you want to get your real date you have to do this:
var date = new Date('3/1/2013');
console.log(date.getDate()+'/'+(date.getMonth()+1)+'/'+date.getFullYear());

Day overflows in javascript Date constructor

I am currently writing some sort of javascript based client calendar and observed some issues. All over the net, I can find code samples, where people use day overflows in the Date constructor.
i.e.
// get the first day of the next month
var myDate = new Date(someDate.getFullYear(),someDate.getMonth(),32);
myDate.setDate(1);
The general idea of this concept is, that since there is no month with 32 days, the constructor will create a date within the next month. I saw even codesample with negative overflows:
i.e.
// get the last day of the previous month
var myDate = new Date(someDate.getFullYear(),someDate.getMonth(),1);
myDate.setDate(-1);
Now while this seems to work in many cases, I finally found a contradiction:
// this prints "2012-12-30" expected was "2012-12-31"
var myDate = new Date(2013,0,1);
myDate.setDate(-1);
Further examination finally revealed that dates like
new Date(2013,0,23) or new Date(2013,0,16) combined with setDate(-1) all end up in "2012-12-31". Finally I observed that using -1 seems to subtract two days (for getting the expected result setDate(0) has to be used).
Is this a bug in the browser implementations or are the code samples spread accross the internet crap??
Furthermore, is this setDate with positive and negative overflow secure to be used and uniformly implemented by all major browsers?

From MDN:
If the parameter you specify is outside of the expected range, setDate attempts to update the date information in the Date object accordingly. For example, if you use 0 for dayValue, the date will be set to the last day of the previous month.
It's logical if you think about it: setDate(1) sets the date to the first of the month. To get the last day of the previous month, that is, the day before the first of this month, you subtract one from the argument and get 0. If you subtract two days (1 - 2) you get the second to last day (-1).
are [..] code samples spread accross the internet crap?
Yes. This is true at least 90% of the time.

At MDN they say:
If the parameter you specify is outside of the expected range, setDate
attempts to update the date information in the Date object
accordingly. For example, if you use 0 for dayValue, the date will be
set to the last day of the previous month.
So you're getting coherent results:
1 - Jan 1
0 - Dec 31
-1 - Dec 30
-2 - Dec 29
Edit: It may look counter-intuitive if you think of it as a mere relative value, such as PHP's strtotime() function:
strtotime('-1 day');
It's not the case ;-)

JavaScript Addition Date Functions

I don't really know too much about core JavaScript, just a dot of jQuery. But I know jQuery is not necessary for what I need here:
I want to use the getdate function to find out the server's day of the week. Then add a bunch of clauses like:
if its Monday add 6 to the date and return the date in MM/DD/YYYY form.
if its Tuesday add 5 to the date and return the date in MM/DD/YYYY form.
if its Wednesday add 4 to the date and return the date in MM/DD/YYYY form.
and so on until Sunday when it will add 0.
So lets say todays Monday, it will return 1/8/2012
And in real dates today's Sunday so it will really return 1/1/2012
Then I just want to call a document.write function to write the MM/DD/YYYY it returns into my HTML document.
Can anybody help me? I can clarify if you need me to...

getDay() returns the day of the week, Sunday = 0, Monday = 1, etc, etc.
So say today was Monday getDay() would return 1, which means daysToAdd would be 5.
Once we know how many days we want to add we can create a new date and add those days. We do this by getting today in milliseconds and then adding the number of days (daysToAdd) in milliseconds.
We convert days to milliseconds by multiplying by 24*60*60*1000 which is the number of milliseconds in a day.
I add 1 to the month because JavaScript returns 0 based month, but for display purposes we want to format it so that January for example is 1 not zero.
function getEndOfWeek() {
var today = new Date();
var weekDay = today.getDay();
// if you want the week to start on Monday instead of Sunday uncomment the code below
//weekDay -= 1;
//if(weekDay < 0) {
// weekDay += 7;
//}
var daysToAdd = 6 - weekDay;
var newDate = new Date(today.getTime() + daysToAdd *24*60*60*1000);
var month = newDate.getMonth() + 1;
var day = newDate.getDate();
var year = newDate.getFullYear();
var formatedDate = month + "/" + day + "/" + year;
return formatedDate;
}
You could implement in your code like so, JavaScript:
$(function() {
$("#TheDate").html(getEndOfWeek());
});
Your HTML would be something like this:
The week ends on <span id="TheDate"></span>.
You can find the jsFiddle here: jsFiddle
If you want to adjust the weekday so that you consider Monday the start of the week instead of Sunday you can do the following after you get the weekDay:
weekDay -= 1;
if(weekDay < 0) {
weekDay += 7;
}

var day = 1000*60*60*24
, nextSunday = new Date(+new Date() + day*(7-((0|(+new Date()/day)%7-3)||7)));
alert(
(101+nextSunday.getMonth()).toString().substr(1) + '/' +
(100+nextSunday.getDate()).toString().substr(1) + '/' +
nextSunday.getFullYear()
)

As fas as adding dates in JavaScipt my "DateExtensions" library does this well enough, I think. You can get it here:
http://depressedpress.com/javascript-extensions/dp_dateextensions/
Once refenced you can call "add()" as a method for any valid date and pass it any of many date parts (second, minutes, days, hours, etc). So assuming "curDate" is a valid JavaScript date object you can add 5 days like this:
newDate = curDate.add(5, "days");
Using a negative value will subtract:
newDate = curDate.add(-5, "days");
Once you get the date you want you can the use the library's dateFormat() method to display it like so:
curDate.dateFormat("MM/DD/YYYY");
There's full documentation at the link.
Integer Values for Day of Week
As for getting the integer value you want, it's actually easier that it looks (and you don't need an "if" just some math). The getDay() method of date returns the day of week with Sunday as "0" and Saturday as "6". So the week, from Sunday, would normally be:
0,1,2,3,4,5,6
First, you want to reverse that scale. That's easily done via subtraction by taking 7 (to total number of members of the set) from the value. This gives you this scale:
-7,-6,-5,-4,-3,-2,-1
We're getting closer. You want the first value to be zero as well. The simplest way (I think) to do this is to get the modulus (remainder) of the value by the total number of members. All this basically does is make "-7" a zero and leave the rest alone giving us this:
0,-6,-5,-4,-3,-2,-1
Almost done. Finally you don't want negative numbers so you need to use the Math.abs() method to eliminate the sign (get the absolute value) leaving us with our desired result:
0,6,5,4,3,2,1
For all the talk the acutual code is pretty compact:
Math.abs((cnt-7)%7)
Wrapping this into the original example gives us:
newDate = curDate.add(Math.abs((curDate.getDay()-7)%7), "days");
Server Vs Client
However take nnnnnn's comment to heart: in JavaScript the getDate() function gets the current date/time of the machine that it's running on - in the case of a web page that's the client, not the server.
If you actually meant the client time them you're set and done. If you really need the server time however that's annoying-to-impossible. If you own the server then it's actually not to hard to set up a rule that includes the current server in a cookie withing each fufilled request (you could then use my cookie library, also at the site above, to access the information!)
It's messier but depending on the server you might also be able to create an old-school server-side include that adds a bit of JavaScript to each page (preferably as a marked replace in the header) that hard-codes the date as a global variable.
You might also create a web service that returns the current server time but the client-overhead for that is insane compared to the data being delivered.
If the server's NOT yours (and you can't get the owner to provide the above) then the only real potential option is to do a straight http call and examine the HTTP "Date" header. Again however the overhead on this is immense compared to the return but it's really the only way. Any system like this would have to be very flexible however as any particular server might not return the date header or might not return it correctly.
Even if it does work understand that you might still not be getting the "server" time - or at least not the server you want. In a tiered architecture, for example an application server might render then page and hand it to a web server to return - you'd be getting the web server time, not the app server. Any number of appliances might also rewrite the headers (for example it's common to use dedicated SSL appliances to offload all the encryption work - these often re-write the headers themselves).
Sorry to get overly technical - JavaScript is definately one area where there's unfortunately rarely a "simple question". ;^)
Good Luck!

How to get difference of saved time and current time in jquery?

I want to get the time difference between saved time and current time in javascript or jquery. My saved time looks like Sun Oct 24 15:55:56 GMT+05:30 2010.
The date format code in java looks like
String newDate = "2010/10/24 15:55:56";
DateFormat format = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
Date date = format.parse(newDate);
How to compare it with the current time and get the difference?
Is there any inbuilt function in jquery or javascript??
Any suggestions or links would be appreciative!!!
Thanks in Advance!
Update
Date is stored as varchar in the DB. I am retriving it to a String variable and then change it to java.util.Date object. The java code looks like
String newDate = "2010/10/24 15:55:56";
DateFormat format = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
Date date = format.parse(newDate);
This date object was sent to client. There i want to compare the saved date with current date and want to show the time difference like 2 secs ago, 2 hours ago, 2 days ago etc... like exactly in facebook. I have gone through some date to timestamp conversion tutorial in java script and now i can get the difference in timestamp. Now, i want to know how i shall change it to some format like "2 secs or 2 days or 24 hours"??. Or, how i shall change it back to date format???

Convert them into timestamps which are actually integers and can get subtracted from each other. The you just have to convert back the resulting timestamp to a javascript date object.
var diff = new Date();
diff.setTime( time2.getTime()-time1.getTime() );

You dont need to explicit convert, just do this:
var timediff = new Date() - savedTime;
This will return the difference in milliseconds.

jQuery doesn't add anything for working with dates. I'd recommend using Datejs in the event that the standard JavaScript Date API isn't sufficient.
Perhaps you could clarify exactly what input and output you're aiming for. What do you mean by "the difference?" There is more than one way to express the difference between to instants in time (primarily units and output string formatting).
Edit: since you said you're working with jQuery, how about using CuteTime? (Demo page)