For what period "down" is calculated?

I had to replace hdd back in December and it took a while to recover and copy old contents, but the “down” number doesn’t change ever since. So it must be percent not “per month”, but per what period then? Doensn’t seem it is per total node age either.

It is based on a rolling 30 days cycle I think.
So say you are down on the 6th of November, you would have to wait until the 7th of December for it to clear the down on the 6th of November.