I have used the this idea from time to time, placing a countdown timer in the sidebar to publicize an upcoming event. The site I have used to construct them is located at TimeandDate.com. The specific page is located here.
Work down the page, selecting such information as the date and time, the style, colors, font size, etc.
Using your information, the site constructs the html code you will need. As you scroll further, you will see how your message will display.
Once your event has passed, if you want to publicize another event, much of you work has been done for you. Return to site where you first built your timer. You should still see the information from your previous event. Edit it, and the html code will be edited also. Copy the new html code, and go to your blog. If you are using the timer in a sidebar, the widget (WordPress) or gadget (Blogger) is already created. Highlight the code that is in it and paste the new code to replace it. If the timer is going in a new blog post, paste the new html code in the post.