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 view & edit older entries?
To modify or review time from over 90 days ago, visit Detailed reports. Use the calendar filter to select a custom date period for your report.
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.