Every connected calendar copied the same span of time, which is fine until one of them is a decade old. A calendar you joined for a single project would drag its whole history into the calendar you actually live in.
Each calendar now carries its own window, set separately for the past and the future: one week, one month, three months, six months, twelve months or two years. The default is one month back and two years ahead, which suits most people without any change. Narrowing a window is not cosmetic — Keeper.sh stops fetching outside it, so a source that was slow because it held years of history gets faster, and events already copied outside the new window are cleaned up rather than left stranded.
Changing a window does not wait for the next scheduled run. Saving it asks for a sync straight away, so widening a calendar to six months of history fills that history in rather than only collecting what arrives from then on.
Read more about what a paid plan adds.