Why can I not create something on a past date?

Clicking a day that has already gone shows Cannot create items on a past date, and the quick-create modal does not open.

This is deliberate. Back-dated items quietly distort everything built on top of them — completion rates, workload, time records — and once that has happened it is very hard to untangle.

If you need to record something that has already taken place, do it in the feature that owns it rather than by planting a new item in the past.