>So for TimeCreated do you mean it is when the activity completion (assignment submission, quiz attempt, etc.) was submitted by a student?
>If I want to know when a grade was recorded in the gradebook do I use the TimeModified value?
From my testing, yes you are correct.
> Why is TimeCreated sometimes null when TimeModified has a value?
I suspect this is when the teacher has graded an assignment/quiz/etc. *before* the student made their draft final and submitted it.