Develop Reference

JS countdown multiple times on page (won't display with getElementsByClassName)

I took some JS code from w3 to create a countdown. (https://www.w3schools.com/howto/howto_js_countdown.asp)
Because I want to display the countdown multiple times on one page I changed the getElementById("demo") to > getElementsByClassName("demo")
Unfortunately, this doesn't work. Nothing shows up. Why is that and how can I display the same counter multiple times? I tried some things but nothing worked out. This is my code:
html
<p class="demo"></p>
js
// Set the date we're counting down to
var countDownDate = new Date("Jan 5, 2022 15:37:25").getTime();
// Update the count down every 1 second
var x = setInterval(function() {
// Get today's date and time
var now = new Date().getTime();
// Find the distance between now and 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.getElementsByClassName("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.getElementsByClassName("demo").innerHTML = "EXPIRED";
}
}, 1000);

As #ShanieMoonlight mentioned you need to iterate over the HTMLCollection. You can easily do it with minimal adjustments. E.g. when you use the spread-operator the forEach-function will be available.
// Set the date we're counting down to
var countDownDate = new Date("Jan 5, 2022 15:37:25").getTime();
// Update the count down every 1 second
var x = setInterval(function() {
// Get today's date and time
var now = new Date().getTime();
// Find the distance between now and 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.getElementsByClassName("demo")].forEach(e => e.innerHTML = days + "d " + hours + "h "
+ minutes + "m " + seconds + "s ");
// If the count down is over, write some text
if (distance < 0) {
clearInterval(x);
[...document.getElementsByClassName("demo")].forEach(e=>e.innerHTML = "EXPIRED");
}
}, 1000);
<p class="demo"></p>
<p class="demo"></p>
<p class="demo"></p>

I need to make a count up timer

I'm trying to make a count up timer, it has to be counting the time that a ticket is getting solved even if you change or refresh the page, when it is over the user has to click the button stop, but I have a problem, when it is counting I press the button Stop and reset it works but when I refresh my site it is still counting and it doesn't stop.
This is for my site:
var countDownDate = localStorage.getItem('startDate');
if (countDownDate) {
countDownDate = new Date(countDownDate);
} else {
countDownDate = new Date();
localStorage.setItem('startDate', countDownDate);
}
var x = setInterval(function() {
var now = new Date().getTime();
var distance = now - countDownDate.getTime();
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);
document.getElementById("demo").innerHTML = days + "d " + hours + "h " + minutes + "m " + seconds + "s ";
}, 1000);
function Reset() {
var countDownDate = localStorage.getItem('startDate');
if (countDownDate == 0, clearInterval(x)) {
countDownDate = new Date(countDownDate);
} else {
countDownDate = new Date();
localStorage.setItem('startDate', countDownDate);
}
}
<button onclick="Reset()">Stop and Reset</button>
<p id="demo"></p>

Making a javascript countdown timer

I am making a countdown timer where the text for Days Hours Minutes Seconds is just below to their respective values. Also it must be responsive too. I have some code below:
// Set the date we're counting down to
var countDownDate = new Date("Jan 5, 2021 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();
// Find the distance between now and 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("timer").innerHTML = "<h1>" + days + " <span> days </span>: " + hours + " <span>hours</span>: " + minutes + " <span>minutes </span>: <font color='red'>" + seconds + "<span> s</span></font> </h1>";
// If the count down is finished, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("timer").innerHTML = "EXPIRED";
}
}, 1000);
<div align="center" id="timer"></div>
My code has a problem in the case that the day symbol D is on left of the Day value but I want it to be on right. I mean just like picture below

You can wrap the text in <div> to create a line break. Secondly create a function which takes text,value and color as parameter and return html string.
// Set the date we're counting down to
var countDownDate = new Date("Jan 5, 2021 15:37:25").getTime();
function timePart(val,text,color="black"){
return `<h1 class="timer" style="color:${color};">${val}<div>${text}</div></h1>`
}
// 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 and 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"
let res = timePart(days,'days') + timePart(hours,'hours') + timePart(minutes,'Mins') + timePart(seconds,'Seconds','red');
document.getElementById("timer").innerHTML = res
// If the count down is finished, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("timer").innerHTML = "EXPIRED";
}
}, 1000);
.timer{
display:inline-block;
padding:10px;
}
<div align="center" id="timer"></div>

Okay so I fixed it according to your requirements. It's not exactly like the picture but I'm sure you can do a little bit of styling. Here is the snippet in action.
var countDownDate = new Date("Jan 5, 2021 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();
// Find the distance between now and 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("dd").innerHTML = days
document.getElementById("hh").innerHTML = hours
document.getElementById("mm").innerHTML = minutes
document.getElementById("ss").innerHTML = seconds
// If the count down is finished, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("timer").innerHTML = "EXPIRED";
}
}, 1000);
h1 span {
margin: 0px 10px;
}
p span {
margin: 0px 11px;
}
<div align="center">
<h1>
<span id="dd"></span>:
<span id="hh"></span>:
<span id="mm"></span>:
<span style="color:red;" id="ss"></span>
</h1>
<p>
<span>Days</span>
<span>Hours</span>
<span>Minutes</span>
<span>Seconds</span>
</p>
</div>

JavaScript count-up timer

I want to add the count-up timer which will count from specified time.
I want to do this as follows:
I would add the button "reset" and after it's clicked the timer starts and counts forever from that specified time, but if I press it again in the future it counts time from that specified time in the future.
var countDownDate = new Date();
// 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.getTime() + 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 ";
}, 1000);
I have code like this but the output is very wrong.
Here is the link: JSFiddle

Use localStorage to save the date
var countDownDate = localStorage.getItem('startDate');
if (countDownDate) {
countDownDate = new Date(countDownDate);
} else {
countDownDate = new Date();
localStorage.setItem('startDate', countDownDate);
}
// 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 = now - countDownDate.getTime();
// 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 ";
}, 1000);
https://jsfiddle.net/5a6ranep/1/

<!doctype html>
<html>
<body>
<button onclick="console.log(getTimeElapsed());">Log Time Elapsed</button>
<script type="application/javascript">
var startTime = Date.now(); // Get Starting time in MS
var endTime = 0;
var timeElapsed = 0;
function getTimeElapsed() {
endTime = Date.now(); // Get current Time
timeElapsed = endTime - startTime; // current time - startTime = Time Elapsed
startTime = Date.now();
return timeElapsed * 0.001; // Convert MS to S
}
</script>
</body>
</html>

Execute code from text input in countdown function

In the following code I want to make a text input field (for a date) which gets executed so that the countdown timer is set to that value and starts counting - for example after clicking "OK" button. I don't really know how to modify the first variable in order to do that.
<script>
// Set the date we're counting down to
var countDownDate = new Date("May 25, 2018 11:30:00").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 = "Pozostało: </br>" + days + " Dni, </br>" + hours + "g : " + minutes + "m : " + seconds + "s";
document.getElementById("demo").style.fontSize = "45px";
document.getElementById("demo").style.fontWeight = "bold";
document.getElementById("demo").style.color = "white";
document.getElementById("demo").style.backgroundColor = "#4783bf";
document.getElementById("demo").style.textAlign ="center";
// If the count down is finished, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("demo").innerHTML = "EXPIRED";
}
}, 1000);
</script>
Thank you for all the advice!

Have a look at this fiddle start timer on click of button
I have entered this date in textbox :- May 26, 2017 01:30:00
function startTimer(){
var dateEntered = document.getElementById("txtDate").value;
// Set the date we're counting down to
//May 26, 2017 01:30:00
var countDownDate = new Date(dateEntered).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 = "Pozostało: </br>" + days + " Dni, </br>" + hours + "g : " + minutes + "m : " + seconds + "s";
document.getElementById("demo").style.fontSize = "45px";
document.getElementById("demo").style.fontWeight = "bold";
document.getElementById("demo").style.color = "white";
document.getElementById("demo").style.backgroundColor = "#4783bf";
document.getElementById("demo").style.textAlign ="center";
// If the count down is finished, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("demo").innerHTML = "EXPIRED";
}
}, 1000);
}
<input type="text" id="txtDate"/>
<br>
<input type="button" value="Calculate" onclick="startTimer();">
<div id="demo">
</div>

The first, you need to download datetimepicker library.
https://plugins.jquery.com/datetimepicker/
And then, following this. Remember to change the path of css and jquery files.
<link href="~/css/jquery.datetimepicker.css" rel="stylesheet" />
<body>
<input type="text" id="datetimepicker" />
<input type="button" value="Ok" id="btOk" />
<p id="demo" />
</body>
<script src="~/lib/jquery/dist/jquery.js"></script>
<script src="~/js/jquery.datetimepicker.min.js"></script>
<script type="text/javascript">
$('#datetimepicker').datetimepicker();
$(document).ready(function () {
$('#btOk').click(function () {
var currentDate = $('#datetimepicker').val();
var countDownDate = new Date(currentDate).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 = "Pozostało: </br>" + days + " Dni, </br>" + hours + "g : " + minutes + "m : " + seconds + "s";
document.getElementById("demo").style.fontSize = "45px";
document.getElementById("demo").style.fontWeight = "bold";
document.getElementById("demo").style.color = "white";
document.getElementById("demo").style.backgroundColor = "#4783bf";
document.getElementById("demo").style.textAlign = "center";
// If the count down is finished, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("demo").innerHTML = "EXPIRED";
}
}, 1000);
});
});
</script>