This very simple app is made to be able to fill in whatever habit challenge you wish to maintain, say studying a foreign language for 20 minutes three time every week, which would become 3 sets of 20 reps per week. Or donate to a homeless person about once a day will become 1 set of 1 rep per day. Then, after any work is done, you update with however many sets or repetitions completed. A running score label shows you the number of sets and reps you are at.
This way you always have motivation to do more and go ahead of schedule as you can see your progress as well as back up for a rainy day later, using the urge to relax later to actually get more work done. Now, having a bad day will not kill the habit as you know exactly how much to do to undo the damage. You also have more motivation to avoid letting even one slip for being able to see your running score decrease.
Or try the full version, Habitifier, with a lifetime supply of habits.