Develop Reference

how to format date pulled from database(inserted as new Date() )

I'm not so sure what's the proper way to insert date into the database, but I'm using new Date().
So I get date format like this when I query from the database:
2021-09-24T12:38:54.656Z
Now I realized that date format is not so user-friendly. So I'm trying to convert it if possible standard readable format like this:
Sept 25 2015, 8:00 PM
I tried using toLocaleString() to my date pulled from db but it won't work probably if I got it correctly pulled date from the db is already a string?
Is there a workaround for this so that I don't need to change how I enter date to my db?

const date = moment("2021-09-24T12:38:54.656Z").format('MMM D YYYY, h:mm a')
console.log(date)
<script src="https://momentjs.com/downloads/moment.min.js"></script>
use momentjs
You can find format method and apply as you desire
https://momentjs.com/

I think it is better to store in DB as UTC 00 value and you are doing it right now.
When you retrieve it, you will be getting a string value something like, 2021-09-24T12:38:54.656Z. On the UI you can easily convert it to date variable in JS using,
const dateVar = new Date("2021-09-24T12:38:54.656Z");
console.log(dateVar.toLocaleString());
If you want more date formatting other than the inbuild solutions like toLocaleString you can use date libraries like moment.js

You'd need to create a Date object from the timestamp string like this.
let date = new Date('2021-09-24T12:38:54.656Z');
Then you can use toLocaleString, toLocaleDateString, toDateString, or toString as you see fit.

How to convert utc to string in momentjs?

I want to use moment for this.
moment("date","YYYYMMDD").fromNow(); // ~~ years ago example
I get UTC from MongoDB createAt. but I don't know convert UTC to string (variable date).
ex) mongodb_createAt = 2018-09-18T12:22:19.491Z
this mongodb_createAt convert to string(date)
I want to utc convert to date string..
I want to display ~~ years ago (like youtube)
How to ??
thanks.

Look at moment documentation https://momentjs.com/docs/#/displaying/fromnow/
Try moment().fromNow(); it should output ... years ago

You could use momentjs's .humanize() method to achieve this.
First, you'll need to calculate the duration between your timestamp 2018-09-18T12:22:19.491Z and the current time. Once you have this duration, you can then make use of .humanize() like so:
var mongodb_createAt = '2018-09-18T12:22:19.491Z';
var millisecondsElapsed = moment(mongodb_createAt).diff(moment());
var displayString = moment.duration(millisecondsElapsed).humanize(true);
console.log(displayString) // eg 17 hours ago
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.2/moment.min.js"></script>
Note that the true argument passed to .humanize(true) adds the tense to the displayString string - ie true causes a result like "17 hours ago" rather than just "17 hours"
For more information on the humanize() method, see the official documentation

How to get hours in moment.js in GMT timezone?

For example if I do for the above date object something like: value.hours(), I get as output 16 instead of 18. I believe it returns the hours in the original GMT time, not like in my date object which is GMT+2. I can of course add 2 to the returned result, but it becomes cumbersome. Is there any way to get the hours correctly in my case?

I'm not sure as to what you've already tried, but I put the following into JSFiddle and it worked like a charm. I am currently in CST in America and it is 8:30 in the morning here. When I ran the snippet below I got today's date at 1:30 PM which I would assume is accurate in difference.
HTML
<div id="m1"></div>
JavaScript
var a = moment.tz(new Date(), "GMT");
document.getElementById('m1').innerHTML = a.format("YYYY MM DD; HH:mm");
The Moment.js documentation states the following in regards to creating a Moment object with a native JavaScript Date object:
You can create a Moment with a pre-existing native JavaScript Date object.
var day = new Date(2011, 9, 16);
var dayWrapper = moment(day);
This clones the Date object; further changes to the Date won't affect the Moment, and vice-versa.
To find the information quoted above quickly, when you reach the Moment.js documentation, it is located under the Parse section under sub-section Date.

To display local time:
value.local();
value.hours(); // 18
To reverse:
value.utc();
value.hours(); // 16

I think that you can solve it by doing what the docs says. Something like this:
moment().tz("America/Los_Angeles").format();
https://momentjs.com/timezone/docs/#/using-timezones/

Format date in JS using Moment.js

There is the following code:
console.log(order.start_time);
console.log(moment(order.start_time).format("HH:MM"));
I just want to get hour and minute from date using moment and display it. Output:
2014-06-30T09:00:00.000Z
13:06
But I don't understand why my date is formatted incorrectly. Thanks in advance.

Please note that there is a difference between MM and mm. The first formatting option is used for months, while the second is for days.
More information here: http://momentjs.com/docs/#/parsing/string-format/

From the doc here:
http://momentjs.com/docs/#/displaying/format/
You should use HH:mm. Then your time will have right format.
Further information, the time might be incorrectly as you expected because timezone. Momentjs auto display time in your system timezone.
Update:
To display time in specified Timezone, use "zone" method:
console.log(moment(order.start_time).zone('+00').format("HH:mm"));

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)