Thanks to Mike Solomon for his timer.
I do not endorse use on flights, the above line was also an April Fools joke. Please, use caution when pulling this prank, especially if you are pulling it on a flight. Airport and Airline security WILL take this prank seriously and you can be either arrested or thrown off the flight at the next airport.
Step 1: Download Timer
The timer is located in the link on this page or below.
Right click on either link and click download, or save link as.
Choose a location, Enter the name or leave as countdown.swf, and hit save.
That is if you just want to save it to your hard drive to pull up later.
Copy this URL for later: http://www.thecleverest.com/countdown.swf and follow more steps if you want to follow the more decorative path.
Step 2: Making the Presentation
Open Impress, choose Blank Template, and clear all of the pre-inserted objects.
Go to Insert>Object>Plug-in...
Insert http://www.thecleverest.com/countdown.swf in the File/URL box and hit OK.
Select the timer, which is small, by clicking and dragging.
When you let go of the mouse button handles for re-sizing the object will appear at the edges and corners. Click and drag these to all four corners/edges. The timer itself won't re-size until a little later, don't worry, it will get larger.