Display and configure a jQuery countdown clock on your sidebar, allowing you to style the result via custom CSS styles.
Download the zip file and extract the contents,
Upload the folder
jquery-countdown-clock to your WP plugin folder
Go to Plugins > Installed, and activate the plugin,
Go to Appearance > Widgets, and drag the widget ("jQuery Countdown Clck") to a sidebar.
Expand the Widget box and, from that, you can change the appearance and display of the countdown timer.