Loading events are events that are triggered when a webpage is being loaded in a browser. There are several types of loading events, including onload, which triggers when all assets on the page have finished loading, and DOMContentLoaded, which triggers when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading. These events can be used to trigger JavaScript code or other actions when the page is fully loaded or when specific elements have been loaded. Here's an example of how to add an onload event to a webpage in JavaScript:

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window.onload = function() {
  // add your code here
};

To hide an element until the page loads, you can use JavaScript and CSS. First, you can set the display property of the HTML element to none using CSS. Then, you can use JavaScript to remove the "display: none" property after the page loads. This can be achieved using the DOMContentLoaded event.

Here's an example using JavaScript:

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// Set the display property of the element to none using CSS
document.getElementById("myElement").style.display = "none";

// Remove the display property after the page loads using the DOMContentLoaded event
document.addEventListener("DOMContentLoaded", function() {
  document.getElementById("myElement").style.display = "block";
});

In this example, replace "myElement" with the ID of your HTML element. The display property is set to "none" to hide the element initially. Then, the DOMContentLoaded event is used to remove the display property and show the element once the page loads.

You can also achieve this using jQuery:

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// Set the display property of the element to none using CSS
$("#myElement").css("display", "none");

// Remove the display property after the page loads using jQuery's $(document).ready() function
$(document).ready(function() {
  $("#myElement").css("display", "block");
});

Again, replace "myElement" with the ID of your HTML element. The display property is set to "none" to hide the element initially. Then, the $(document).ready() function is used to remove the display property and show the element once the page loads.

There could be several reasons why you can't select an element by ID. One possible reason is that the ID you are trying to select does not exist in the HTML code. Double-check the HTML file to ensure that the ID you want to select is spelled correctly.

Another possible reason is that the script running the code is being executed before the elements in the HTML file have fully loaded. To solve this issue, you can wrap your code in a DOMContentLoaded event listener, which will ensure your code is run only after the page has fully loaded.

Here's an example using JavaScript:

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document.addEventListener("DOMContentLoaded", function() {
  let currentDate = document.querySelector("#currentDayTime");
  // rest of your code goes here
});

If you're still having trouble selecting an element by ID, try selecting it by another attribute like class or tag name, as a temporary workaround.