Unhappy [Q] When was my phone rooted?

Hi all,

Is there a way to find out when was my phone rooted? Actual date?
I bought a phone from a small private store. Lately I got an OTA update to 4.4.2 KitKat. I updated the phone and it failed starting after this update. I returned to the store with a complain. After a month!!!! of tests with no result the guy from the store blames me that he does not cover the malfunction repair because the phone was rooted unoficially before the update.
So in order to prove that I did not root my phone I need to find out somehow when the phone was actually rooted.
Is this even possible?
P.S: I think the KNOX is tripped... did not have a chance checking it.