Thread: Where should I start

It is certainly doable, but you need to consider a few things:
1. You need to familiarize yourself with the Android API (obviously) and Java (basic java skills is probably all you'll need for this)
2. Where do you want to "save" your time information? Do you want to save it to your device, save it to some sort of online storage, ect.?
3. Do you want the program to get the time based on the current time, or do you want to be able to adjust the time (for example, let's say you were on the clock but didn't get around to starting the app until 5-10 minutes later for some reason)?

Break your problem up into different program flows. For example:
Primary Flow:
1. Log in, get time, save time as start time
2. Log in again, get time, save time as end time (alternatively, you can keep the program running all day and record the time when you close it, but that seems like a waste of phone resources)
Secondary Flow 1:
** Some sort of way to track when you are covering for someone
Secondary Flow 2:
** Some sort of way to print the information (if you want to print to a specific form, that can probably be done, but it will most likely require some creativity. It depends on the form.)

What you have is a good start. But before you try to do anything, I suggest you hash out in extreme detail specifically what you want to do, how you want to store the data, ect. Also, get a piece of paper and draw out how you want your user interfaces to look. It is easier to sort it out on paper before trying to redesign it in code.