When we want to know precisely when Google, Wayback (or any other robot which features cache copies of our code), we implement a spider-spotting "cache stamp" using server environment variables, specifically DATE_LOCAL to match time information with any cache copy out there. This helps us with forensics including copyright protection. There are many ways to implement a cache stamp and we simply use Server Side Includes (SSI) directives to echo the environment variable.
The time-format statement (config timefmt):
%a %b %d %y, %I:%M %p %Z
The first part (before the comma) formats the way day, month and year will display. The second part formats the way time appears. Statements asking for the DATE_LOCAL environment variable will return local server time. We host in Pittsburgh with Pair Networks and use EST at the end to keep everything consistent for accuracy when reading a remote cache copy. It's important to understand this information can be hidden inside the code itself, which is how we might implement it with a client who wouldn't want the information visible on their pages. In our case, it's not a problem to display it as part of our footer.