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Will there be any issues with my countdown timer?

This code for a countdown timer is taken from W3Schools:
// Set the date we're counting down to
var d = new Date();
var n = d.getTimezoneOffset() * 60000;
var countDownDate = new Date("May 2, 2018 15:54:25").getTime();
// Update the count down every 1 second
var x = setInterval(function() {
// Get todays date and time
var now = new Date().getTime();
// Find the distance between now an the count down date
var distance = countDownDate - now - n;
// Time calculations for days, hours, minutes and seconds
var days = Math.floor(distance / (1000 * 60 * 60 * 24));
var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
var seconds = Math.floor((distance % (1000 * 60)) / 1000);
// Output the result in an element with id="demo"
document.getElementById("demo").innerHTML = days + "d " + hours + "h "
+ minutes + "m " + seconds + "s ";
// If the count down is over, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("demo").innerHTML = "EXPIRED";
}
}, 1000);
There was a big issue with it: any date I enter would not work for different timezones, as if it was +1 hour UTC it would expire, and it would be an hour long if it was -1.
So I added the d.getTimezoneOffset() * 60000; to get the milliseconds difference, and then took it off, so if there is any difference it should calculate it.
Will it always work if I put an UTC date in the future, or is there a possible difference I don't know about (for example a special country or daylight savings that could mess it up)?

create countdown based on server hour

I´m facing a need of creating a coundown to display when ever a certain promo will be over, all i have is front-end access via google tag manager, first i manage to get the server hour with js, now i need to set the countdown hour based on the server hour and not local hour.
<script>
// it´s giving the correct hour
var getserver = new Date();
getserver.setHours(getserver.getHours());
var newtets = getserver.toUTCString();
alert(newtets);
// using a basic countdown
var countDownDate = new Date("Sep 5, 2018 15:37:25").getTime();
// Update the count down every 1 second
var x = setInterval(function() {
// Get todays date and time
var now = new Date().getTime();
/*
var now = new Date();
now.setHours(now.getHours());
var newnow = now.toUTCString();
i tried to replace now with newnow but is not working, how can i transform this
*/
// Find the distance between now an the count down date
var distance = countDownDate - now;
// Time calculations for days, hours, minutes and seconds
var days = Math.floor(distance / (1000 * 60 * 60 * 24));
var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
var seconds = Math.floor((distance % (1000 * 60)) / 1000);
// Output the result in an element with id="demo"
document.getElementById("demo").innerHTML = days + "d " + hours + "h "
+ minutes + "m " + seconds + "s ";
// If the count down is over, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("demo").innerHTML = "EXPIRED";
}
}, 1000);
</script>
var now = new Date().getTime(); is what getting the time and use it to countdown, this is taking local time, how can i get var now = new Date().getTime(); to be server hour, which i obtained with the 1 script

The only way to get your updated server hour is using an ajax call.
The problem is that the request to the server is not immediate so you will have a delay.
Maybe it would be better to get the user timezone and calculate the promo ending with this information.
You can get the user timezone offset using:
var offset = new Date().getTimezoneOffset();

showing countdown timer difference on different systems

I am trying to make a Countdown Timer in javascript and I wrote a code like
var countdown = function(){
setInterval(function() {
var countDownDate = new Date(document.getElementById("end_date").getAttribute("data-date")).getTime();
// data-date ex. = "2017-11-28 21:54:00"; greater than current date (now)
// Get todays date and time
var now = new Date().getTime();
// Find the distance between now an the count down date
var distance = countDownDate - now;
// Time calculations for days, hours, minutes and seconds
var days = Math.floor(distance / (1000 * 60 * 60 * 24));
var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
var seconds = Math.floor((distance % (1000 * 60)) / 1000);
// Display the result in the element with id="demo"
if(hours<10){
hours = "0"+hours;
}
if(minutes<10){
minutes = "0"+minutes;
}
if(seconds<10){
seconds = "0"+seconds;
}
var left = hours + ":"+ minutes + ":" + seconds;
console.log(left);
document.getElementById("time_left").innerHTML = left;
// If the count down is finished, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("time_left").innerHTML = "EXPIRED";
}
}, 1000);
}
countdown();
Counter is working fine but why I am getting difference of time upto 25 secs on different systems. Some systems shows same countdown time but, some not.

you should use your sever time and java script take the system time so when u change your system time count down will change automatically. you can also provide me your code using js fiddle or anything else...

Countdown Timer using php to activate a button click

Need a help guyz, my scenario is - In my database i am having a table with a column "created_time" which stores the current system time in HH:MM AM/PM. Now I want as soon as the data entered in that table with current system time then in one of my php page there will be a button named "Start Exam", currently the button will be in "disabled" state, it should be enabled exactly after 1 hour of the time entered in the database along with a countdown timer showing along with the button.
Can anyone help me out with that.

<script>
var countDownDate = new Date("<?php echo $dateFromDB ?>").getTime();
// Update the count down every 1 second
var x = setInterval(function() {
// Get todays date and time
var now = new Date().getTime();
// Find the distance between now an the count down date
var distance = countDownDate - now;
// Time calculations for days, hours, minutes and seconds
var days = Math.floor(distance / (1000 * 60 * 60 * 24));
var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
var seconds = Math.floor((distance % (1000 * 60)) / 1000);
// Display the result in the element with id="demo"
document.getElementById("demo").innerHTML = days + "d " + hours + "h "
+ minutes + "m " + seconds + "s ";
// If the count down is finished, write some text
if (distance < 0) {
clearInterval(x);
// do something when time is expired
}
}, 1000);
</script>

setInterval js does not update result

I have my countdown function (that works).
// Set the date we're counting down to
var countDownDate = new Date("<?= $stop_datejs ?>");
// Update the count down every 1 second
var x = setInterval(function() {
// Get todays date and time
var now = new Date();
// Find the distance between now an the count down date
var distance = countDownDate - now;
// Time calculations for days, hours, minutes and seconds
var days = Math.floor(distance / (1000 * 60 * 60 * 24));
var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
var seconds = Math.floor((distance % (1000 * 60)) / 1000);
// Output the result in an element with id="demo"
document.getElementById("demo").innerHTML = days + "d " + hours + "h "
+ minutes + "m " + seconds + "s ";
// If the count down is over, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("demo").innerHTML = "ACTUALIZA LA PÁGINA";
}
}, 1000);
But I need to compare with a php variable $actual_date (server date)
// Get todays date and time
var now = new Date("<?= $actual_date ?>");
That works but stops updating every second. What is the problem?
Thanks

The variable now will not be updated, because the variable will only be written once. So if you load the the webpage the value will be set once but not updated, because the website does not update again (Only your interval).
To provide this you can simply use:
// Get todays date and time
var now = new Date();
This will only use the current time for the new instance of Date. If your <?= $actual_date ?> is not the current timestamp or you want to sync if for all browsers/pc with an incorrect time setting you should have a look at AJAX. Using AJAX is the easiest way to get the updated time from the backend.
jQuery Ajax:
http://api.jquery.com/jquery.ajax/