Countdown Timer

I would like to build an app that counts down by days. This would be a stand alone app that can be used on a desktop, smartphone or tablet. (Windows or Mac) I would like the app to be generic so that a user could enter in text data for each count down day that would automatically open for the amount of days that the user decides. Of course the entry could be the same for ALL days OR could be modified where the user could enter different data for each day. For example, a user could enter data for a 30 day exercise workout routine. The user enters data and time for each day of the workout and a message pops up each day at the designated time reminding the user that they should exercise NOW and a text and/or audio cue would play telling them what they need to do for TODAY'S exercise. I am sure there are apps that do this already but I would like to have an app that I OWN and can design different count down uses for various types of tasks. I would like to give some of the versions for free and sell the more interesting uses that I create. I am familiar with Actionscript 3 and so I know this could be done with Flash, but Flash is no longer cross-platform. So, I am looking for suggestions of the proper type of programming that should be employed to create this app with the features I have stated.