Please can you help me. I have a number of projects with forecast benefits, I need to calculate the difference between latest months benefits and original months benefits, the problem is that not every project starts in the same month. I have attached a workbook (Tableau v 8.3), this shows the projects, the benefits and the months, I need to add a calculated field so that I can show difference between latest months benefits and the original benefits/forecast. The difficult bit is that some projects have benefits that start in January but other projects start later in the year. Please can you let me know if it possible to add a calculation which can calulcate current month minus original month irrespective of what the original month is?