In order to keep things running super fast, the maximum period that a report can cover is 6 months or 10,000 messages (whichever is larger).
If you require a larger report, for example, if you'd like to run a report for a year, you need to run 2 reports (one for the first 6 months, another for the last 6 months). You can then email these reports to yourself and collaborate the information together.
For reports that are over 10,000 messages, the report will be run overnight.
Also, we have improved the experience by not allowing you to enter an end date prior to a start date when filtering by time/date.