I suspect that since the 9th century CE was 1200 years ago, he added an extra hundred years in the wrong direction when thinking about the 15th century (i.e., h

I suspect that since the 9th century CE was 1200 years ago, he added an extra hundred years in the wrong direction when thinking about the 15th century (i.e., he thought 1427 was in the 14th century, which is seven centuries before the 21st century).