Theory:Doctor Who prose discontinuity and plot holes/The Tomorrow Windows


 * Apparently in the eighth century the third princess Tabetha of Cerrenis Minor once spent a weekend on Earth. Only there was no 'eighth century' at the time. The BC/AD calendar was only introduced in what we today call the tenth century AD. And at the time, every country, kingdom or region had their own calendar, none of which correspond to the modern dating system.
 * I don't see the relevance, as the historian would have been using the BC/AD (or the BCE/CE) calendar to retroactively date the royal's visit.