I've used a countdown widget in OS X for ages, but recently, I wanted a program that could keep track of several events to count down to, and something specifically for my iPad, not just for my Mac.
I found a universal app simply called Countdown, and I figured that for $1, I'd give it a try. It turned out to be exactly what I was looking for, plus a little more.
InterfaceThe concept is fairly simple and so is the interface. You are asked to name the event and provide a date (time is optional). That's all you have to tell it, but there's more available. You can also choose the "Units" of measurement. This was one of the crucial features for me; If I have a deadline for a project coming up on March 1, I want to see how many days I have to work on it. By default, Countdown will show you year, month, day, hour and minute, so that my March 1 deadline is shown as "2 months, 13 days." However, if I want to see just the number of days left, I can uncheck all of the "Units" except "Day," and it will show "75 days" remaining.
If I wanted to show just weekdays (53 remaining) or just weekend days (22 remaining), I can do that as well. On the other hand, I am on a bicentennial committee, and that deadline isn't until March of 2015, so I don't need to see "days" for that. Instead of days, I have years and months listed until it gets closer. Finally, I'm going to Chicago in April, and I don't need to see how many days that is, but I would like to know how many weeks (16!). You can group events by folder if you want (if you do, however, there is no way to see all of your events at once), and you can set them to "auto-sort," where they will organize themselves chronologically.