I'm surprised no-one has said this yet.
The time in your email is the email received time. This email was sent by Google in response to your form submission, so is by definition sent after your submission was received by some unknowable time interval. Servers are fast, but aren't instantaneous, especially considering the sudden spike of high load. This means that if your email time is 20:30:00 then it's possible your form was actually received at 20:29. Likewise, if your email time is 20:33:03, it's possible your form was actually recorded on 20:33:02 or 20:33:01.
With such a tight 2-second cutoff, there's a high chance that the email time is inaccurate for many people. The only way to know for sure it to see the filled spreadsheet, or just wait for your invoice.