Android : Guidance For Structuring App / Change Image - Data To Display

I am a new to developing for Android and have been reading up and researching in order to become more familiar with the platform. My programming knowledge is moderate I have experience with C++ and a fairly proficient with Actionscript 3.0. I picked up programming primary to work on game development but come from a more artistic/design background.

I have a relatively simple idea for an app that I want to approach from the most practical way possible seeing how it's my first attempt. Essentially the core functionality of the app will be drawing information I will be getting from an ephemeris chart. At a certain time in the day which is based on the data from the chart I want to change an image/display information based on that day. I am trying to wrap my head around the best possible way to do this. Also I would like to give the user an option of having a notification of some kind when the change occurs, but that is something I have come across in terms of basic implementation.

So my question is if you were to approach setting something like this up how would you? What I am not sure about is setting up a time element so the application knows when to change the image/data to display.