By default, sudo displays a lecture when the user's time stamp file is not present. In sudo 1.6, the -k option was changed to reset the time stamp file to the epoch rather than remove it to prevent the lecture from being displayed the next time sudo was run. No special case was added for handling a time stamp file set to the epoch since the clock should never legitimately be set to that value.
However, there are two common ways for the clock to be reset to the epoch. The first way is when the clock is reset due to a fully drained battery on some systems. The other way is by a user logged in to a desktop environment that allows changes to the date and time.
As long as the user has successfully run sudo before, they are able to run "sudo -k" to reset the time stamp file. This action does not require a password and is not logged. If the user is also able to reset the date and time to the epoch (1970-01-01 01:00:00), they will be able to run sudo without having to authenticate.