Bolivia 90 DAYS tourist visa: per any 365 days, or per calendar year??

Hola, I have what I imagine to be quite a simple question, but one which seems not to have been answered on this forum yet (and isn't mentioned on the Immigration Office page).

Is the 90 DAY LIMIT on tourist vias/s per ANY 365 DAY PERIOD, or per CALENDAR YEAR??

Any (accurate!) help would be greatly appreciated! :-)