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Countdown timer in nodejs

I am creating a website for students which will be used to assign exams and I am having difficulties with the timer. The one I am using is made on the frontend in javascript and whenever the page is refreshed the timer startsover. Tried to store the start and end date by converting to epoch and back to datetime but I cannot think of a way to get the timer to the frontend and start counting. The idea is to count 60 minutes and call the submit button as well as to show the countdown without the option to restart the counter.
This is how I store the start and end time in nodejs.
var myDate = new Date();
var startTimeEpoch = myDate.getTime()/1000.0;
var endTimeEpoch = startTimeEpoch + 5400 // Adding 90 minutes to the timer
var startTimeBackToDate = new Date(startTimeEpoch *1000)
var endTimeBackToDate = new Date(endTimeEpoch *1000)
This is the javascript timer I am using and I am wondering if I should use one in the first place.
function startTimer(duration, display) {
var start = Date.now(),
diff,
minutes,
seconds;
function timer() {
diff = duration - (((Date.now() - start) / 1000) | 0);
minutes = (diff / 60) | 0;
seconds = (diff % 60) | 0;
minutes = minutes < 10 ? "0" + minutes : minutes;
seconds = seconds < 10 ? "0" + seconds : seconds;
display.textContent = minutes + ":" + seconds;
if (diff <= 0) {
start = Date.now() + 1000;
}
}
timer();
setInterval(timer, 1000);
}
window.onload = function () {
var fiveMinutes = "<%= scenario.time %>" * 60,
display = document.querySelector('#time');
startTimer(fiveMinutes, display);
}

As a general response and with the additional information provided, i could propose a solution to make this work.
If your students all have a specific exam entity attached to them, when they register/start an exam, you could retrieve the start date of this exam(add a mongo createdAt Date field) and use it as the starting date.
If each exam has a time limit, then you could simply do the math to know how much time is left. Something that will look like this:
const getExamRemainingTime = (exam) => {
// assuming that start is a js date object
// and timeLimit is an number representing the duration hours of your exam
const { start, timeLimit } = exam;
let end = (start.getHours() + timeLimit);
end = end.setHours(end);
const remainingTime = (+end) - (+start)
if (remainingTime > 0) {
// duration not finished, exam still in progress
return new Date(remainingTime);
} else {
// exam finished
return 0;
}
}
Then in your frontend, if it's plain javascript, you need to refresh your timer component, use setInterval in last ressort because it's very heavy on performance and format the date you got the way you want to show it.
Ref: casting js Date object to timestamp - How do you get a timestamp in JavaScript?.

I don't think a timer that a student with Javascript knowledge can modify should be used for serious tests, but for anything more light-hearted it should be fine.
The best system I can think of for this would be to have the test length stored in the mongodb and when a signed-in user starts the test, have the current time logged for that user. That way, you can calculate time remaining using user.testStart + test.length - Date.now().

calculating time difference in iso time format in javascript?

I have two date string in ISO format. I am calculating the minutes difference between them using moment js.
var currenttime = new Date().toISOString();
var expiretime = '2020-06-05T12:18:33.000Z';
let minutes = moment(expiretime, 'YYYY-MM-DD[T]HH:mm:ss. SSS[Z]').diff(moment(currenttime, 'YYYY-MM-DD[T]HH:mm:ss. SSS[Z]'), 'minutes');
console.log(minutes)
Now i have two questions
Is this is the best way to calculate the minutes difference ?
When i run new Date().toISOString() the value before z is the supposed to be the timezone but on every restart it changes ?
Please let me know what is the issue ?

I don't think you have to include moment for this purpose only, this can be done in ordinary Javascript.
To calculate time difference just subtract the two timestamps which will give the difference in milliseconds. Then divide by 60*1000 which will give the time difference in minutes.
let currentTime = new Date();
let expireTime = new Date('2020-06-05T12:18:33.000Z');
let minutes = (expireTime - currentTime) / (1000 * 60);
console.log(minutes);

For your first question, what you can do is simply subtract the two dates (date objects and not strings) ,
Newer date - Older date ( this will give the result in milliseconds)
or use Math.abs() function to calculate milliseconds -
var msec = Math.abs( currenttime - expiretime );
and convert this to minutes using -
var min = Math.floor((msec/1000)/60);
2)
Z in the datetime string stands for Zulu, it indicates that the time is in UTC format.
You can simply subtract two date objects.
var currenttime = new Date();
var currenttime_UTC = currenttime.getUTCDate();
var expiretime = new Date('2020-06-05T12:18:33.000Z');
var msec = Math.abs( currenttime_UTC - expiretime );
var min = Math.floor((msec/1000)/60);

Converting to duration from milliseconds

I am trying to convert time duration from the format of mm:ss.mss to entirely milliseconds and back.
I've already have a working function for converting from milliseconds to duration but I cannot seem to get it the other way around.
Lets say for instance that I have the duration 32:29.060, I want to convert it to milliseconds. For that I use this function:
function millisecondsToTime(ms, digits) {
digits = digits || 12;
return new Date(ms).toISOString().slice(23-digits, -1);
}
var a = millisecondsToTime(5549060, 9);
but whenever I try to convert back to time duration, I fail. I've tried parsing individually the minutes, seconds and milliseconds but it doesn't seem to work.
Here is the code that I've used for it:
var firstSplit = a.split(':')
var minutes = firstSplit[0]; //1
var secondSplit = firstSplit[1].split('.');
var seconds = secondSplit[0]; //2
var millisec = secondSplit[1]; //3
var conversion = ((+minutes) * 60 + (+seconds) * 60 + (+millisec))*1000;
I have an input bar which takes the format of mm:ss.mss and I need to convert it to milliseconds. How can I do that?

you can just return a
new Date(ms)
to get a date from ms.
And to get the same date as ms,
date.getTime() // returns ms from date object
Full example:
const ms = 5549060
const date = new Date(ms) // get a date from ms
console.log(date.getTime) // logs 5569060
If your input is a string in the format of mm:ss.mss, and you want to get a date from it, you can use moment.
const moment = require('moment')
const date = moment('22:15.143', 'mm:ss.SSS') // get date from pre specified format

You can use the string methods indexOf() and substr() to get the individual numbers out of your string and calculate the time accordingly.
I'm afraid though your millisecondsToTime() function isn't working properly.
5549060 milliseconds are roughly 92 minutes and it's returning 32:29.060
function backToTime(time) {
var index = time.indexOf(":");
var minutes = time.substr(0, index);
var seconds = time.substr(index + 1, time.indexOf(".") - (index + 1));
var milliseconds = time.substr(time.indexOf(".") + 1, time.length);
return parseInt(minutes * 60 * 1000) + parseInt(seconds * 1000) + parseInt(milliseconds);
}
console.log(backToTime("32:29.060"));

Your conversion to milliseconds is not working, this is basic math approach to both conversions:
let input = 5549060
//toDuration
let seconds = Math.floor(input / 1000);
let ms = input - seconds*1000;
let m = Math.floor(seconds / 60);
let s = seconds - m*60;
duration = m + ":" + s + "." + ms
console.log(duration)
//toMilliseconds
let holder = duration.split(":");
m = parseInt(holder[0]);
holder = holder[1].split(".");
s = parseInt(holder[0]);
ms = parseInt(holder[1]);
milliseconds = (m*60 + s)*1000 + ms
console.log(milliseconds)
If needed add check for ms length to add 0s, if you need it to have length of 3

I think your milliseconds to duration converter will be broken for durations above 60 minutes. This is because using Date the minutes field will wrap over into the minutes after 59 seconds have passed. If you want to get good support for values beyond 59 in your first field, I think maybe moving to a regex-based parser and using multiplication and addition, division and modulo to extract and reduce the fields manually might be nice. Something like this maybe:
var duration = ms => `${(ms / 60000) | 0}`.padStart(2, '0') + `:` + `${ms % 60000 / 1000 | 0}`.padStart(2, '0') + `.` + `${ms % 1000}`.padStart(3, '0')
var millisec = durat => (match => match && Number(match[1]) * 60000 + Number(match[2]) * 1000 + Number(match[3]))(/^([0-9]+)\:([0-5][0-9])\.([0-9]{3})$/.exec(durat))
You can see given the input 5549060, this function provides output 92:29.60, which is exactly 60 seconds greater than your own, and I believe to be correct. Maybe it's intentional for your usecase, but I can't imagine that being so desirable generally...

Real Time decreasing value in php

I need some advice and logic in my problem.
So, I have an entrydate, from database, then the running current date, and a value of 10(double type in database). So, I know how to calculate the diff of the entrydate and current date, right. So I convert it to seconds then to a number(9.23165).
|Entry |Current Date|Diff(in number)|
|2:00:00 PM |2:30:00 PM | 5.00(Sample)|(First User)
So basically, as current date goes on, can PHP show the deduction on real time? Or I need to refresh? What I need is for it to display the deduction without refreshing. So basically, I need to know what I have to do. Maybe javascipt and ajax?

What you would need are a few Javascript/jQuery functions to update the browser in real time.
var myTimer;
var startTime;
function startTimer() {
stopTimer(); // Reset
startTime = new Date(); // Save to calculate difference
myTimer = setInterval(clockTicking, 1000);
}
function stopTimer() {
clearInterval(myTimer);
}
function clockTicking() {
var now = new Date();
var timeDiff = new Date(now - startTime); // constructor uses UTC, so use UTC date functions from here on
var hours = (timeDiff.getUTCHours() < 10) ? '0' + timeDiff.getUTCHours() : timeDiff.getUTCHours();
var mins = (timeDiff.getUTCMinutes() < 10) ? '0' + timeDiff.getUTCMinutes() : timeDiff.getUTCMinutes();
var secs = (timeDiff.getUTCSeconds() < 10) ? '0' + timeDiff.getUTCSeconds() : timeDiff.getUTCSeconds();
$("<element-where-you-display>").html(hours + ':' + mins + ':' + secs);
}
In Javascript you can call startTimer() to kick it off.

Jquery time difference in hours from two fields

I have two fields in my form where users select an input time (start_time, end_time) I would like to, on the change of these fields, recalcuate the value for another field.
What I would like to do is get the amount of hours between 2 times. So for instance if I have a start_time of 5:30 and an end time of 7:50, I would like to put the result 2:33 into another field.
My inputted form times are in the format HH:MM:SS
So far I have tried...
$('#start_time,#end_time').on('change',function()
{
var start_time = $('#start_time').val();
var end_time = $('#end_time').val();
var diff = new Date(end_time) - new Date( start_time);
$('#setup_hours').val(diff);

try
var diff = ( new Date("1970-1-1 " + end_time) - new Date("1970-1-1 " + start_time) ) / 1000 / 60 / 60;
have a fiddle

It depends on what format you want your output in. When doing math with Date objects, it converts them into milliseconds since Epoch time (January 1, 1970, 00:00:00 UTC). By subtracting the two (and taking absolute value if you don't know which is greater) you get the raw number of milliseconds between the two.
From there, you can convert it into whatever format you want. To get the number of seconds, just divide that number by 1000. To get hours, minutes, and seconds:
var diff = Math.abs(new Date(end_time) - new Date(start_time));
var seconds = Math.floor(diff/1000); //ignore any left over units smaller than a second
var minutes = Math.floor(seconds/60);
seconds = seconds % 60;
var hours = Math.floor(minutes/60);
minutes = minutes % 60;
alert("Diff = " + hours + ":" + minutes + ":" + seconds);
You could of course make this smarter with some conditionals, but this is just to show you that using math you can format it in whatever form you want. Just keep in mind that a Date object always has a date, not just a time, so you can store this in a Date object but if it is greater than 24 hours you will end up with information not really representing a "distance" between the two.

var start = '5:30';
var end = '7:50';
s = start.split(':');
e = end.split(':');
min = e[1]-s[1];
hour_carry = 0;
if(min < 0){
min += 60;
hour_carry += 1;
}
hour = e[0]-s[0]-hour_carry;
min = ((min/60)*100).toString()
diff = hour + ":" + min.substring(0,2);
alert(diff);

try this :
var diff = new Date("Aug 08 2012 9:30") - new Date("Aug 08 2012 5:30");
diff_time = diff/(60*60*1000);