The Toggl Calendar View is limited to displaying no more than 3 months (90 days) of data. The same limit applies to the Timer page list view and is due to performance reasons.
How to create entries with a past date?
To create a time entry that has a start date more than 90 days (3 months) into the past, use the time input strip at the top of the page instead of the actual Calendar UI.
For specific details on how to perform these steps, open the Manual Mode article.
How to view & edit older entries?
The Detailed report page supports inline editing. With the report generated, click on the part of the time entry you want to edit (description, project, time, etc) and perform the changes.