There will be two unit test methods
1. first will check whether InterpreterCostAuthorisedDate is set with current time or not / you can check whether date field is null or not
Assert.IsNotNull(InterpreterCostAuthorisedDate)
2. second will check whether InterpreterCostAuthorisedDate is set null or not
you can assert it using
Assert.Isnull()
Hope this will help you.