The person with the excellent diagram also noted something along these lines with the timestamp idea. The problem with the timestamp idea, and giving .3 seconds leeway to all AoE abilities before they actually hit you is twofold:
1 - You should get IMMEDIATE feedback whether you have been hit or not hit
2 - What you see on your screen should be the definitive version of the AoE and your position in it

If you add in these artificial delays to the AoE "hit", then we end up with a stuttery experience in terms of when we see actual damage. If these ideas were implemented, then you wouldn't know if you got hit by the AoE until .3 seconds after it looked like you got out of it. So basically, you're going to watch yourself die OUTSIDE of the AoE sometimes. Think of the fall damage delay we already experience.