Timing ends on both circumstances yes, however it's easier to tell when time ends when someone doesn't star cancel, than when they do, and since it doesn't really matter, it's not hurting anyone taking it away.

Why are you asking exactly?

In what way? Timing ends on the VERY FIRST FRAME of you grabbing the star.

The rule is mainly in place to differentiate between Stage RTA and Stage RTA (SC), with the obvious observation being you can star cancel in one, but not the other.