Historical Counting, aka HC, is the standard counting of years in Yurpsland.
From the author's note for Monday, September 28, 2015(
):
HC stands for "Historical Counting"; at some point a group of wise humans somewhere determined the age of the Wide World to be XXXX years, the local potentate started using it as his dating system, it spread, and by the time it was found they'd gotten it completely wrong, (it's much more than 5000-odd years) it was too established to easily change. So it just got a new label. This system has only been used universally in "Mansionland" since the country was unified under Yorik I.
At the time Rosemary arrives at The Mansion of E the year is 5423 HC.