Firstly, this topic is in no way meant to insult anyone's intelligence. Nuances of interpretation can be a real gray/grey area. In case anyone is confused about this incident, I want to provide a simple, brief explanation of the math. Again, if this seems obvious, it means that you already interpreted things the "right" way.
The key lies in the terms "not before" and "not after" along with the one-second granularity of digitally accounting for time. Many of us are perhaps more used to the terms "begins at" and "expires at". When accounting by the second, "not before" and "begins at" noon have the same meaning. When the clock transitions from 11:59:59 AM to 12:00:00 PM, the period starts. However, "not after" and "expires at" one do not have the same meaning. When the clock transitions from 12:59:59 PM to 1:00:00 PM, the period has "expired at one" and thus the period lasted one hour. However, when the clock transitions from 12:59:59 PM to 1:00:00 PM, the time is still "valid" within the period because the time is invalid "after" 1:00:00 PM. Since we are using digital time granularity, the period expires at 1:00:01 PM, which is the first granular unit of time meeting the condition of being "after" one. Thus, the period lasted for one hour and one second.