Here's an alternative method: Create numeric data type temp001, calculated field is JUL(2016,12,13)-10
Then, if you want to subtract 10 days, in the data type string (calculated) field you want the result: DTE(Temp001)
The first formula in the temp field converts the date to a julian numeric value, then subtracts 10 days. If you want to use todays date use JUL(0,0,0). The second formula converts the julian numeric value back to a date.