For example, lets say I set a goal to walk for 1 hour per day. But then I only walk for 30 minutes, how would the app handle that situation?
Why did you want to track the goal in the first place? This is probably the most important question.
Why didn’t you set the goal to 30 minutes, if there are days where it’s likely that you miss your more ambitious goal?
If you set the goal to walk an hour per day and you just walk 30 minutes, you obviously failed to reach the goal. Depending on the first two questions, you can be hard on yourself for this or not.
Because the goal is still an hour, but there are different levels of failing. A simple binary "pass/fail" means the record shows the same difference between lying in bed all day Vs running a marathon and walking 59 minutes Vs 61. If I'm 2 grams above my goal weight at Christmas is that a simple "fail"?
You are right that setting a goal aligned with what you want to change is the important question, but you want you achieve may not well fit into a clear pass/fail.
You could make this percentage-based. Track the progress, show green or some kind of achievement only when you have 100%.
I’d argue that this system is okay too, but more complicated than a simple binary yes/no choice.
Walking 59 minutes or 2 grams above goal weight: If you are serious about the goal, this could serve as a great reminder that you should push through the last minute of walking or get down on Christmas and burn an extra 150 kcal.
Again, depends on how serious you are about your goals.
Unfortunately at the moment it is binary (done or not-done).