In general it's a best practice to provide a packaged workbook or some sample data. This will increase the chances of getting help, particularly if related to some aggregation or data relationship. In your case I think the question is pretty simple and the screenshot and your description should be enough.
Try, If YEAR([Accounting date]) = 1900 THEN 'PLAN' ELSE STR(YEAR([Accounting Date])) END. Use this on colors and as your filter.