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.
OptionsThere are several other options as well: you can set an image for the event, which is handy if you have a bunch of them and want to find one at just a glance. Several images are included, or you can use one from your photo library. There are also options for repeating events, and you can set a reminder notification using either the deadline itself or another date/time that you select. This makes Countdown a handy little timer as well, if you need to know when it is 5:00 PM but don't necessarily want to add it to your iCal or your iPhone's default clock app. Local notifications are used for iOS 4 devices. According to the app's descriptions, notifications are also supported in iOS 3, although I haven't tested that myself.
Although the app is clearly named "Countdown," you can also set events in the past, and the app will tell you how long it has been since a certain date/time. For example, I have been alive for 13,776 days, and my son is currently 8 years, 6 months, 17 days and 7 hours old.
In-App PurchasesThere is a $3 in-app purchase available if you want to sync Countdown between iOS devices (i.e., your iPad and iPhone). That isn't a feature that I needed, but it's nice that it is offered, and nice that it's a separate charge to keep the app price low. You only need to purchase the sync once, not per device.
ConclusionAll in all, this is exactly what I was looking for, and more. I have seen it get confused a few times (when adding an event, the label names were listed askew), but I simply relaunched the app, and the problem went away. Also the auto-sort sometimes takes a few seconds to kick in, which is a very minor nit. If you're looking for an app like this, check it out.