Combine 2 different date fields into 1 Date

how can I create a date variable that combines order date from the sales header and order date from the sales invoice header so when my report runs I only see 1 date on the report? Example I have an order date of 12/20/16 in Sales header and an order Date of 12/01/16 in Sales invoice header so on the report I would like to see just the month of December representing both dates.