The Lightweight Countdown Timers plugin lets you create simple countdown timers and place them anywhere with a shortcode. The plugin ads a timers section in the main menu. Here you can create multiple timers with a few simple settings. When saved, the shortcodes to show the timer in the frontend are showed.
Additionally you can use a shortcode to show content only before, or after the deadline of the timer.
The timers always use the timezone set in your WordPress setting, keep this in mind when setting the deadline.