WIY: It’s a Machlokes Rishonim when Shmittah is. The Rambam has a calculation but disregards it in lieu of a tradition that the Geonim had.
We know when Shemittah was, in theory, because we know when B’nei Yisrael entered Eretz Yisrael. However, there is a Machlokes Tannaim whether Yovel counts as the first year of the next Shemittah cycle or not. To know when Shemittah is we would have to know that the Beis Din HaGadol never Paskened the way that we don’t Pasken now (or that they corrected it post facto if they ever did). That’s just one of the issues with knowing when Shemitah is. We assume we know, but we are not completely sure.