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Date and Time Difference and Average time

I have two times (basically start time and end time). Also, I have the number of questions played by the user. I wanna know the average time user spent for each question.
//startGameTime: 2019-07-27T07:58:42.000Z
//endGameTime: 2019-07-27T07:59:57.000Z
function averageQuestionTime(startGameTime, endGameTime, totalNumberOfQuestions) {
var d1 = new Date(startGameTime);
var d2 = new Date(endGameTime);
var d1msecs = new Date(d1).getTime(); // get milliseconds
var d2msecs = new Date(d2).getTime(); // get milliseconds
var avgTime = (d1msecs + d2msecs) / totalNumberOfQuestions;
var date = new Date(avgTime);
var hour = date.getUTCHours();
var min = date.getUTCMinutes();
var sec = date.getUTCSeconds();
var day = date.getUTCDate() - 1;
return (day + ":" + hour + ":" + min + ":" + sec)
}
I understand my logic is completely flawed as the units for date and time and nos of questions are different. What is the best way to achieve the result ?

There are good libraries which prevent the users from having to reinvent the wheel every time they want to manipulate date/time in Node.
Obtaining time difference is pretty simple (I can see you are doing it correctly to obtain the difference in milliseconds) and libraries make them even simpler.
See how simple it is using momentJS
var moment = require('moment');
var startDate = moment('2019-7-24 00:00:00', 'YYYY-M-DD HH:mm:ss');
var endDate = moment('2019-7-24 05:27:31', 'YYYY-M-DD HH:mm:ss');
var diffSeconds = endDate.diff(startDate, 'seconds');
var diffHours endDate.diff(startDate, 'seconds');
console.log(`Avg in secs: ${diffSeconds / totalNumberOfQuestions}`);
console.log(`Avg in secs: ${diffHours/ totalNumberOfQuestions}`);

calculate difference between two timestamp and get difference in unix timestamp

I want to calculate the difference between two dateTime, one date is submitted by user and other is current time:
user submitted time - now = difference in unix
user submitted time format is:
2014-03-26 10:52:00
Thanks for your help.

You can simply do this with getTime() which returns the number of milliseconds.
var ds = "2014-03-26 10:52:00";
var newDate = new Date(ds).getTime(); //convert string date to Date object
var currentDate = new Date().getTime();
var diff = currentDate-newDate;
console.log(diff);
Sometimes there are chance for cross browser compatibility in parsing the date string so it is better to parse it like
var ds = "2014-03-26 10:52:00";
var dateArray = ds.split(" "); // split the date and time
var ds1 = dateArray[0].split("-"); // split each parts in date
var ds2 = dateArray[1].split(":"); // split each parts in time
var newDate = new Date(ds1[0], (+ds1[1] - 1), ds1[2], ds2[0], ds2[1], ds2[2]).getTime(); //parse it
var currentDate = new Date().getTime();
var diff = currentDate - newDate;
console.log(diff); //timestamp difference

You can use MomentJS library
var user_submited_time = moment('2014-03-26 10:52:00');
var now = moment();
var value = user_submited_time - now;

JavaScript / Extjs - Calculate number of hours from time stamp

I have this time stamp format for each car in my map:
2012-12-11T03:51:43+03:00
I want to extract the number of hours from it according to current time.
I don't know how to parse this string then compare it to current time.
Any Idea ?

something like:
var
d1 = new Date('2012-12-11T03:51:43+03:00'),
d2 = new Date;
console.log(
(d2 - d1) / 3600000
);

You need to first fix the timestamp for other browsers than Chrome
javascript date.parse difference in chrome and other browsers
DEMO
var noOffset = function(s) {
var day= s.slice(0,-5).split(/\D/).map(function(itm){
return parseInt(itm, 10) || 0;
});
day[1]-= 1;
day= new Date(Date.UTC.apply(Date, day));
var offsetString = s.slice(-5)
var offset = parseInt(offsetString,10)/100;
if (offsetString.slice(0,1)=="+") offset*=-1;
day.setHours(day.getHours()+offset);
return day.getTime();
}
alert(parseInt((new Date().getTime()-noOffset(yourTimeStamp))/3600000))

How do you get the unix timestamp for the start of today in javascript?

I realize that the current timestamp can be generated with the following...
var timestamp = Math.round((new Date()).getTime() / 1000);
What I'd like is the timestamp at the beginning of the current day. For example the current timestamp is roughly 1314297250, what I'd like to be able to generate is 1314230400 which is the beginning of today August 25th 2011.
Thanks for your help.

var now = new Date();
var startOfDay = new Date(now.getFullYear(), now.getMonth(), now.getDate());
var timestamp = startOfDay / 1000;

Well, the cleanest and fastest way to do this is with:
long timestamp = 1314297250;
long beginOfDay = timestamp - (timestamp % 86400);
where 86400 is the number of seconds in one day

var now = new Date; // now
now.setHours(0); // set hours to 0
now.setMinutes(0); // set minutes to 0
now.setSeconds(0); // set seconds to 0
var startOfDay = Math.floor(now / 1000); // divide by 1000, truncate milliseconds

var d = new Date();
d.setHours(0);
d.setMinutes(0);
d.setSeconds(0);
d.setMilliseconds(0);
var t = d / 1000;

Alternatively you could subtract the modulo of a days length in miliseconds e.g.
var day = 24*60*60*1000;
var start_of_today = Date.now() - Date.now() % day;

Luis Fontes' solution returns UTC time so it can be 1 hour (daylight saving time) different from setHours solution.
var d = new Date();
var t = d - (d % 86400000);
Simplified version of examples above (local time).
var d = new Date();
d.setHours(0,0,0,0);
var t = d / 1000;
Here you can find some performance tests: http://jsperf.com/calc-start-of-day

Another alternative for getting the beginning of the day is the following:
var now = new Date();
var beginningOfDay = new Date(now.getTime() -
now.getHours() * 60 * 60 * 1000 -
now.getMinutes() * 60 * 1000 -
now.getSeconds() * 1000 -
now.getMilliseconds());

var yoursystemday = new Date(new Date().getTime()-(120000*60+new Date().getTimezoneOffset()*60000));
yoursystemday = new Date();
var current_time_stamp = Math.round(yoursystemday.getTime()/1000);

For any date it's easy to get Timestamps of start/end of the date using ISO String of the date ('yyyy-mm-dd'):
var dateString = '2017-07-13';
var startDateTS = new Date(`${dateString}T00:00:00.000Z`).valueOf();
var endDateTS = new Date(`${dateString}T23:59:59.999Z`).valueOf();
To get ISO String of today you would use (new Date()).toISOString().substring(0, 10)
So to get TS for today:
var dateString = (new Date()).toISOString().substring(0, 10);
var startDateTS = new Date(`${dateString}T00:00:00.000Z`).valueOf();
var endDateTS = new Date(`${dateString}T23:59:59.999Z`).valueOf();

var now = new Date();
var startOfDay = new Date(now.getFullYear(), now.getMonth(), now.getDate());
var timestamp = startOfDay.getTime() / 1000;

Strange bug in this time calculation script?

Basically this script will subtract StartTime from EndTime, using a jQuery plugin the html form is populated with Start and End Time in the format HH:MM, an input field is populated with the result, it works except for one issue:
If Start Time is between 08:00 and 09:59 it just returns strange results - results are 10 hours off to be precise, why?
All other inputs calculate properly!
function setValue() {
var startTime = document.getElementById('ToilA');
var endTime = document.getElementById('EndHours'); startTime = startTime.value.split(":");
var startHour = parseInt(startTime[0]);
var startMinutes = parseInt(startTime[1]);
endTime = endTime.value.split(":");
var endHour = parseInt(endTime[0]);
var endMinutes = parseInt(endTime[1]);
//var hours, minutes;
var today = new Date();
var time1 = new Date(2000, 01, 01, startHour, startMinutes, 0);
var time2 = new Date(2000, 01, 01, endHour, endMinutes, 0); var milliSecs = (time2 - time1);
msSecs = (1000);
msMins = (msSecs * 60);
msHours = (msMins * 60);
numHours = Math.floor(milliSecs/msHours);
numMins = Math.floor((milliSecs - (numHours * msHours)) / msMins);
numSecs = Math.floor((milliSecs - (numHours * msHours) - (numMins * msMins))/ msSecs); numSecs = "0" + numSecs; numMins = "0" + numMins; DateCalc = (numHours + ":" + numMins);
document.getElementById('CalculateHours').value = DateCalc; }

Whenever you have math problems with the number 8, it's something getting converted into the octal system :)
Numbers starting with 0 are interpreted as octal numbers in Javascript.
It's no problem from 01..07, because they are the same in both systems.
But 08 and 09 don't exist in the system, so they return 0.
Also see this question that also provides a solution: Specify the base parameter when doing the parseInt:
parseInt("09", 10); // base 10