https://sfdo-docs.s3-us-west-2.amazonaws.com/npsp_reports.pdf
Reporting snapshots in Salesforce allow you to capture and store data at specific intervals, providing a way to track changes over time. However, they do not work retroactively, meaning you cannot use them to capture historical data before the snapshot was created. The snapshots only start capturing data from the point they are set up moving forward.
To address the development director's request, the consultant should consider alternative methods, such as creating reports with historical data that has already been captured or using a different tool that can analyze historical data for trends.
Reference:
Salesforce Help: Create Reporting Snapshots to Report on Historical Data Salesforce Trailhead: Reporting Snapshots