You can set the use Business calendar boolean property to false, so that it wont use the Business Calendar while adding the days to date. As per the addDays(String startDate, int daysToAdd, boolean useBusinessCalendar, String calendarName) below description is provided,
addDays function adds daysToAdd to startDateTime and returns the date String.
If startDateTime is blank or null, we default to currentDateTime.
If useBusinessCalendar is set to true, but no calendarName is specified, 'Default' calendar is used. If it fails obtaining calendarCache for the calendarName, or if any exception happens while adding business or calendar time to a startDateTime then also 'Default' calendar is used. If useBusinessCalendar is set to false, then a Calendar is obtained with the GMT timeZone and adds daysToAdd by setting Calendar time to startDateTime.