How to know the date of upgrade from 6.0 to 6.9.1?

We've got upgraded from 6.0 to 6.9.1 few months back. I need the exact date of the upgrade so that I can get it as an evidence for my internal reports. Is there anyway I can find it in logs or anywhere else?

Please note that we've restarted ACM and AFX server couple of times after the upgrade. Not sure if that really matters but just wanted to put it here.

Thanks in advance,

DG