Yes it's the only we can verify the times of the kills regardless of time-zone.
Actually, achievements do change timestamps according to your timezone so it's possible to verify solely via ingame. Something I tested out of curiosity.
Example: Get achievement at 8 PM Eastern. Change timezone of personal clock back an hour, the achievement will say 7 PM. This holds true for viewing other people's achievements as well, so it's possible to just have someone make a character and look at a person's achievements to verify.
Regardless though, thank you for keeping track of the progression.