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get time difference between a ISO date and date now?

So i am trying to get how many seconds there between a ISO date and current time.
My attempt looks like this:
var date = new Date('2018-01-20T15:45:16.698Z').getTime();
var now = Date.now();
var timeLeft = date - now;
var timeLeftInSeconds = Math.floor(timeLeft/ 1000);
but i am getting an incorrect difference and i assume it becuase that ISO dates are UTC while Date.now() are local time. but now sure how to work around this issue?

How do I calculate hours from PM to AM using moment.js?

I've been working on calculating the total hours, minutes and seconds between two times using the moment.js library. The problem I have is that the calculations don't recognise that it's a day and return more hours than what is there. I'll give you an example:
I want to know how many hours are between 21:00 and 06:00, the answer is 9, however, the calculation brings back -15, this is also technically correct. I need to tell moment that it should use a day to calculate this value. I can get it to work if I use a DateTime picker but I don't want to use that as the user is only required to provide a time.
My application uses KendoUI for MVC and moment.js, moment.duration.format.js and moment.range.js
Here is my code which will return -15
#(Html.Kendo().TimePicker().Name("start").Value("21:00"))
#(Html.Kendo().TimePicker().Name("end").Value("06:00"))
<button class="btn btn-default btn-success" onclick="calc()">Plot</button>
Here is the javascript that works with this.
function calc() {
window['moment-range'].extendMoment(moment);
console.clear();
var dformat = "HH:mm:ss";
var start = $("#start").data("kendoTimePicker").value();
var end = $("#end").data("kendoTimePicker").value();
var startTime = moment(kendo.toString(start));
var endTime = moment(kendo.toString(end));
var duration = moment.duration(endTime.diff(startTime));
var hours = parseInt(duration.asHours());
console.log(hours);
}
If we change this to use DateTimePicker instead, it understands there is a day and calculates 9 hours. So how do I get around this? How can I achive this without using a datetime picker? Can I leaverage moment.js startof day or something?

Thanks to #VincenzoC I have managed to fix this problem. Here is the code which checks if the end time is before the start time and if it is, add a single day. This means the resulting time is accurate.
var startTime = moment(start);
var endTime = moment(end);
if (endTime.isBefore(startTime))
{
endTime.add(1, 'd');
}
//
//After the above condition has been passed, calculate the difference
var duration = moment.duration(endTime.diff(startTime));
//
//Any format you want
console.log(duration.format("HH"))

calculate minutes from starting and ending time

I am trying to calculate how many minutes a worker works from the input starting and ending time(e.g. 10:30 am to 3:30pm). Could u guys help how to calculate them? Could u check my code and correct them? I am very new in Javascript.
function myFunction(){
var sTime=document.getElementById("startTime").value;
var eTime=document.getElementById("endTime").value;
var diff = sTime-eTime;
var result= diff.getMinutes();
document.getElementById("demo").innerHTML=result`;
https://jsbin.com/bolapox/edit?html,output

You will need to turn the users input into a usable format with Date().parse(input). This returns the number of milliseconds since 1 January, 1970, 00:00:00, local time.
You can then take the difference in milliseconds and convert them into minutes.
var sTime=Date().parse(document.getElementById("startTime").value);
var eTime=Date().parse(document.getElementById("endTime").value);
var diff = eTime - sTime;
var result = diff / 60000;

You should consider Moment.js, here yiou can find some examples:
http://momentjs.com/docs/#/durations/

Trouble with .getTime()

I am fairly new to HTML and Javascript, so I'm trying to make a small incremental game as practice. This is the code I am trying to use to calculate the automatic gains / second, as well as adjust accordingly for when the tab isn't in focus and setInterval stops running.
var startTime = new Date();
var endTime = new Date();
var interval = 100;
window.setInterval(function(){
startTime.getTime();
var timeDiff = startTime - endTime;
do{
document.getElementById('woodAmount').innerHTML = Math.floor(user.wood += (user.WPS/10));
document.getElementById('oilAmount').innerHTML = Math.floor(user.oil += (user.OPS/10));
document.getElementById('goldAmount').innerHTML = Math.floor(user.gold += (user.GPS/10));
document.getElementById('coalAmount').innerHTML = Math.floor(user.coal += (user.CPS/10));
timeDiff -= interval;
}while (timeDiff >= interval);
endTime.getTime();
}, interval);
For some reason, this code doesn't adjust for the time when the tab is not focused, but it works as expected when it is in focus.
As you can see here, I set the interval to 100 milliseconds, and I divide the resources / second (user.WPS) by 10.
However, when I set the interval to 1 second (1000 milliseconds) and don't divide the resources / second by 10, it works as expected all the time, and properly adjusts for the time that the tab isn't focused.
Can anyone offer an explanation as to why it works when using full-second intervals, but won't when using 100 millisecond intervals?

.getTime() gets the time that is already in the Date object at the time it was created or whenever the time was last set in the date object. It does NOT get the current time.
If you want to get the current time, I often use this little function:
function now() {
return new Date().getTime();
}
Or, if you don't need IE8 support, then you can use Date.now().
In addition, the getTime() method pulls the time out of the data object and returns it from that method call. If you want to use it, you have to put it somewhere after calling .getTime().

javascript unixtime one minute in front

Hi I'm passing a unixtimestamp to a javascript IF statement, can anyone tell me how to generate a unixtimestamp one minute in the future with javascript.
Anyhelp would be helpful.
Thanks

The JavaScript Date object has a getTime() method that returns milliseconds since 1970. To make this look like a UNIX timestamp, you need to divide by 1000 and round (with Math.floor()). Adding 60 get's your one minute ahead.
var d = new Date();
var unixtimeAdd60 = Math.floor(d.getTime()/1000)+60;

UNIX time is just the number of seconds since 1970-01-01Z. So just add 60 you'll get a timestamp one minute later.

JavaScript Date object's getTime returns the number of milliseconds since midnight Jan 1, 1970.
Try this.
var oneMinLater = new Date().getTime() + 60 * 1000;
var d = new Date();
d.setTime(oneMinLater);

Another way to get the unix timestamp (this is time in seconds from 1/1/1970) in a simple way its:
var myDate = new Date();
console.log(+myDate + 60); // you just sum the seconds that you want
// +myDateObject give you the unix from that date