State changes are shown as "Maturity History for each object on it's info pages and available Query Builder as Life Cycle History type, with the timestamp when the object reached a certain state as the "Created" date of that event.
Join Life Cycle History to your object type.
Use fixed criteria:
- Object.state = (select the state you care about)
- Object.Action = Enter_Phase
Enter date range of interest when running the report.
Results will be the timestamps when each object reached the state within the date range. On the select tab, include the object.