You must have Admin Level - Webmasters access to add a countdown timer.
How to Create Your Countdown Timer
Try one of these sites:
http://www.timeanddate.com/clocks/freecountdown.html
https://countingdownto.com/ (has a watermark)
Or Google "countdown timer embed" to find one to your liking. Make sure it doesn't use Javascript.Customize your timer as you see fit.
Find the area or button to generate the HTML code.
Select the code in the box and copy it (Ctrl+C in Windows, Command+C on a Mac, or right-click and select Copy).
How to Embed the Counter on the Site
If you want to put the timer on a user-defined page,
In the side menu, click Org or Team Resources > Manage Org Resources.
Click the by the desired page.
On the right, click the < > Source... button.
If you want it on the home page,
In the side menu, click Website Design.
Click the Main Text overlay (or add a Generic Component and click that).
Put the cursor where you want the timer to appear and paste the code you copied above (Ctrl+V Windows, Command+V Mac, or right-click and select Paste).
If you want the timer centered, add
<p style="text-align: center">before the code and</p>after the code (see example at the bottom of this article).Click Update.
Click Save.
You may need to refresh the page to see the timer.
If you add paragraph tags to center the text (step 4 above), it will look something like this:
<p style="text-align: center">
<iframe src="https://free.timeanddate.com/countdown..."></iframe>
</p>