Obviously it's octal and the person is a time traveler from the 11th century.

It's the long now foundation thing. The long now foundation encourages writing years with five digits to encourage readers to think about long term planning, to plan for a future of humanity that is measured in more than thousands of years.

https://en.wikipedia.org/wiki/Long_Now_Foundation

<https://news.ycombinator.com/item?id=45505856>

<https://news.ycombinator.com/item?id=43463920>

<https://news.ycombinator.com/item?id=39175614>

They want to feel like they matter in over 10k years from now, where a 4-digit year would start to wrap.

In fact that will be not even 8k years from now.

I’ll be very embarrassed when I’m still writing 9999 on my checks.