The Reporting Module - Timesheets Data Provider will only show data back 18 months from the time the upgrade to 16.3.3 was done (and Time Slicing Sync job was run.)
16.3.3
Current design upon upgrade to 16.3.3 was to include the SQL curves for time entry to only go back 18 months from upgrade time.
Current plans for 16.4.1 include the ability to customize the timeframe for time entries included for MUX reporting
Refer to Reporting Module - Timesheets Data Provider - Does not show timesheets that have never been opened from the UI for periods only open for time entry showing available for Reporting.