Applying a date filter to a date report can lead to confusing bucketing in combined reports.
1) Create two combinable start/end date reports.
2) Add a start date filter for one of the reports that results in an empty report, for example "start date last year" in a start date report where all instances were started this year.
3) Create a combined report with both reports.
The combined report includes all buckets of the report that actually has data, the report that has no data should have no buckets.
The result includes the correct buckets for the report without a date filter as well as empty buckets (according to the unit, e.g. Hour) for the entire last year up to today, see screenshot.