Set a countdown timer with Power Nap
There are many ways that you can set an alarm with webOS, such as using the stock alarm feature within the Clock app, creating a calendar entry, or using apps like Timer Classic or Dashboard Utilities to set a countdown timer. However, if you are looking for an app that is both extremely simple to use yet also offers you a plethora of features and preferences, you should check out Power Nap, by Kevin Foreman. For 99 cents in the App Catalog, Power Nap's intended purpose was to help you set a time to wake you up from a power nap, but it can be used for almost any purpose.
Upon launching the app, you are given a few buttons and a slider that will allow you to quickly set the minutes or hours for your power nap, while displaying both the duration of the timer and the alarm time. You can actually set the default nap time in the app's preferences (accessible by swiping down from the top-left to view the Application dropdown menu and selecting Preferences), along with settings for your alarm ringtone, if you want vibration on or off, a default snooze time, and more. Once you have you desired alarm time set, tapping the "Set Nap Time" button then brings up another pop-up that lets you set the alarm volume (which can be independent of the system's volume setting) as well as an option to show a countdown screen in the app card or just give you the dashboard notifications.
The two features of the app that really set this apart from the other methods or time apps is that even if you close both the app and dashboard, the timer still runs in the background (to access the dashboard controls in the future, just relaunch the app), and the vibration alarm has a randomizing algorithm that makes the device seem to almost go crazy when the timer goes off. This is actually intentional, to help the "napper" realize an alarm is going off. So, the next time you need to set an alarm, whether it's for 15 minutes or 15 hours, check out Power Nap.
Power Nap is available in the webOS App Catalog for $0.99 and is compatible with all webOS devices running webOS 1.3.1 or higher