640 months is a little over 53 years, so like 640 months ago would be Thursday, January 15, 1970 and 640 months from today be Tuesday, September 15, 2076. I will be 75 in 640 months and my parents would have been tiny children 640 months ago.
640 months ago would correspond to January 1970. January 1, 1970 is the Unix Epoch date and is a commonly used format for tracking time differences. If the server reported its Unix Timestamp as 0 because of a technical issue, you would get 640 months.