SQL date calculation

SQL date calculation

I'm trying to calculate the 20th day of the month following a column 'DueDate' in a query table.

In SQL, this works:
  1.  DATEADD(d,19,DATEADD(m, DATEDIFF(m, -1, DueDate), 0))
But in Zoho, it returns this error:

Length of the display column name 'DATE_ADD(DATE_ADD(0, INTERVAL  PERIOD_DIFF(EXTRACT(YEAR_MONTH FROM so.duedate), EXTRACT(YEAR_MONTH FROM - 1))  MONTHS), INTERVAL  19  DAY)' exceeds 100 characters. Length of the display column name should be less than 100 characters.

Can anyone recommend an alternative?