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Kotlin JavaScript Date.now() return type

I was wondering why the Kotlin JavaScript Date class returns a Double for the getTime function. According to the documentation, the getTime function should return the number of milliseconds since 1 January 1970 00:00:00 UTC.
I know that JS doesn't have a 64 bit numeric representation, but since Kotlin emulates Longs I feel like the value returned by Date.now() and Date().getTime() should be a Long. At the very least it would make more sense to return an Int.
Is there any reason that it returns a Double instead of a whole number?

There are two separate reasons for that: consistency and performance.
But before I get into that please note that you can easily get the desired behavior via extensions, e.g.:
inline val Date.time get() = getTime().toLong()
println(Date().time) // 1522757176433
Consistency. As you can see in the Date documentation page, it an external class. Which means it just describes an existing JavaScript API. Is it a good design decision to reuse it as it? Maybe not. But it has a benefit of being familiar to the JS folks. Changing the return type of a single function ruins that and makes the whole API quite inconsistent.
Performance. It is possible hide the original getTime and create a helper function instead. But keep in mind that in order to emulate Long Kotlin creates an object with 2 Number's. Creation, comparison, binary operations, storage - emulated Long's perform a lot worse than native JS Numbers.
Summing up. Changing the return type would make the API inconsistent and inefficient. If you value semantics over performance, just write a few helper functions.
P.S. I think the Date API will be redesigned at some point to make it possible to use it in multiplatform projects. But that's another story.

In kotlin use Date().time .It will return the long Values you will get whole number
val s= Date().time
print(s)
for example
val date = "01-02-2018 07:05:00.999"
val fmt = SimpleDateFormat("MM-dd-yyyy HH:mm:ss.S") //parse date based your format
var myDate: Date? = null
try {
myDate = fmt.parse(date)
} catch (e: ParseException) {
e.printStackTrace()
}
println(myDate)
val timestamp = myDate!!.time //timestamp values in long only not double
println(timestamp)

Add o'clock/Uhr to a globalize.js formatted date

Pretty simple question: I want to add a switch to my globalize format skeleton, so that it adds an "o'clock" or German "Uhr" after the time. Right now, I'm using the following pattern:
yMMddHHmm
I found the o'clock string in the respective cldr files, but I don't know how to add them to the skeleton.
Here's my code:
Globalize.formatDate(dTermin, { skeleton: 'yMMddHHmm' });
Thank you in advance for your help!

There's no skeleton that would give you "o'clock" kinda output [1]. Although, if you really want that format and are willing to provide the i18n data for all the locales you think could support it you can use globalize's raw option like:
// Not recommended anyway...
if (locale === "en") {
formatter = globalize.dateFormatter({raw: "HH 'o''clock'"});
}
Though, it's usually a not recommended approach since you have to maintain this list of custom formatters yourself.
1: In order to check, I've grep'ed the whole CLDR and found no match.

Calculate date difference in jQuery

The question is; there are 2 fields in my application, one is date (Field1) and second is a label (Field2). So, I want that when user selects a date in field 1, then field 2 should be automatically populated (current date - date from field 1).
Can anyone help on how to implement it.
I'm using jQuery to display date:
// This displays the date dialog when user clicks on Field1
$('#Field1').click(function () {
$('#Field1').simpleDatepicker();
});
// Tried following code but it didn't worked
$('#Field1').click(function () {
$('#Field1').simpleDatepicker({
onSelect: function () {
$('#Field2').value(calculateDays($('#Field1').toString))
}
});
});
function calculateDays(dateString) {
var today = new Date();
var inputDate = new Date(dateString);
var days = today - inputDate;
return days;
};
This may look like pathetic code to some folks but I'm just a beginner, so any suggestions/comments are welcome.
Also please tell me if this can be done using html only and no need to go to jQuery. It is my understanding that the calculating days (difference between dates) code will go in jQuery since this needs to be fired after selecting date ('onSelect' event). Please correct if wrong.

I'm assuming that you're trying to use Karl Seguin's jquery.simpleDatePicker (it came top when searching for "simpledatepicker" on Google).
As Jimbo remarks in the comments, it's hard to advise on an MVC approach here — you say you want to do this purely with HTML, but HTML alone can't dictate behaviour (I'd say that's extremely un-MVC). HTML5 forms do allow some limited behavioural control (validation etc), and they also offer <input type="date"/>, but none of these help your situation.
So for this answer I'm just going to fix the mistakes in your code:
The plugin is initialised with the simpleDatePicker jQuery method — you forgot to capitalise the 'P';
The plugin itself caters for the click event. You should initialise it directly without waiting for user input;
There was no onSelect initialisation option in the source code: I chose to use a change event listener on the input to capture this;
You use the jQuery method value — that's native DOM Javascript — you should be using val instead;
toString won't work on DOM elements or jQuery objects — again, use the val method;
The native Date object can't parse dates in arbitrary formats — nor would your code produce a number of days if it did (it would just produce the difference in milliseconds). For this kind of functionality you should use a good date library: I've opted for Moment.
Resulting code (as demonstrated here):
$('#Field1')
.simpleDatePicker()
.on('change', function passValue(){
$('#Field2').val(calculateDaysFromNow($('#Field1').val()))
});
function calculateDaysFromNow(dateString){
return moment.duration(moment(dateString,'MMM DD YYYY').diff()).days();
}
A bit of elaboration on how I've used moment:
First of all, we want to parse #Field1's formatted date for an actual quantifiable date object:
moment(dateString,'MMM DD YYYY')
Next, we want to differentiate that from now. Like Date, moment assumes now if we pass no argument:
moment(dateString,'MMM DD YYYY').diff()
We don't want this as a date, but as a duration, so we'll pass it to moment's duration method:
moment.duration(moment(dateString,'MMM DD YYYY').diff())
…and finally, we want this expressed in days:
moment.duration(moment(dateString,'MMM DD YYYY').diff()).days()

I'm not sure but this:
$('#Field2').value(calculateDays($('#Field1').toString)) should be like this:
$('#Field2').value(calculateDays($('#Field1').val())) or $('#Field2').value(calculateDays($('#Field1').text()))

Here is solution for setting same date in second field.
Link:jquery: using two datepicker with two fields ( field 1 , field2 + 1 day ) like booking.com
Change the format according to your need.

Recurring events on a calendar : RFC 5545 Javascript parsing

I need to integrate recurring events into an adapted version of full-calendar that has an added javascript module which allows offline event browsing.
I'm looking for a javascript library that can parse recurring events according to RFC 5545.
I need to be able to list all recurring events that occur between 2 dates (start date and end date), using RRULE and EXDATE and interpreting daily, weekly, monthly and yearly recurrences.
I've spent hours searching for something to no aval, and I don't want to reinvent the wheel....Can anyone please point me in the right direction for an existing javascript parser?

I checked into skyporters rrule_parser and found that it doesn't support all of the rules (particularly, it won't do BYDAY properly). I found a fantastic alternative:
https://github.com/jakubroztocil/rrule
They are actively supporting this library and have a great demo website that shows all of the functionality. You can parse from either 5545 format or plain text (using the nlp extension). It is feature packed and, as far as I can tell, fully functioning.

look into https://github.com/skyporter/rrule_parser.
I hope it will help you.

here is a recurrence widget for jquery, which parses/creates RFC5545 compatible recurrence strings.
https://github.com/collective/jquery.recurrenceinput.js
it does not expanding of a recurrence rule into occurrence dates, though. but it includes a python server, which can do it for you, using python-dateutil: http://labix.org/python-dateutil

I needed this functionality myself, along with time zone support, so I made a typescript/javascript library: rSchedule.
Currently supports all ICAL recurrence rules except BYSETPOS, BYWEEKNO, and BYYEARDAY. Supports serialization to/from ICAL format along with a ton of extra stuff.
Example:
const rule = new RRule({
frequency: 'YEARLY',
byMonthOfYear: [2, 6],
byDayOfWeek: ['SU', ['MO', 3]],
start: new Date(2010,1,7),
}, {
dateAdapter: StandardDateAdapter
})
let index = 0;
for (const date of rule.occurrences()) {
date.toISOString()
index++
if (index > 10) break;
}
rule.occurrences({
start: new Date(2010,5,7),
take: 5
})
.toArray()
.map(date => date.toISOString())

Parse ONLY a time string with DateJS

I'm using the excellent (but large) DateJS library to handle dates and times in my webapp. I just came across something that I'm not sure how to handle.
I want my users to be able to enter Time strings only, without a date, but they should be able to enter it in any manner they please. For instance:
5:00 pm
17:00
5:00pm
5:00p
5p
etc.
Using Date.parse(value) converts these strings into a full date, which is exactly what I want. However, it also allows the user to enter any other part of a date string, such as:
sat 5pm
1/1/2010 5pm
etc.
I'm trying to use DateJS to validate an input field for a time value. Something like:
function validateTime(value) {
return Date.parse(value) !== null;
}
Is there a way to use DateJS features to solve this? There are other SO questions that provide solutions, but if DateJS has a way to do this, I don't really want to add more custom code to my app to do this.

Shortly after asking my question, I discovered that Date.parseExact() can take an array of format strings. Somehow I'm missed that. I managed to get something working with the following code:
function validateTime(input) {
return Date.parseExact(input, [
"H:m",
"h:mt",
"h:m t",
"ht","h t"]) != null ||
Date.parseExact(input, [
"h:mtt",
"h:m tt",
"htt","h tt"]) != null;
};
Note that some formats don't seem to be able to be included together at the same time, which is why I split them into two separate parseExact() calls. In this case, I couldn't include any string that contained a single t in it with format strings that contained a double tt in it.

The additive approach seems cumbersome. Takes away the beauty of DateJS in my opinion. I needed the same solution and decided to just sneakily append the date in front of my input string before parsing with DateJS:
var parsed = Date.parse(Date.today().toString('M/d/yyyy') + ' ' + this.value);
if (parsed) {
alert(parsed.toString('h:mm tt'));
}
Now DateJS will not be sniffing around for any of its date-part parsing patterns, as you have already subbed it in.
Hope this helps someone!