I have used the addToDate function to take 14 days away from a date property in my application.  

E.g. I used this syntax in a data transform to set the date 14 days before the .DateProperty.

@DateTime.addToDate(.DateProperty,-14,0,0,0)

You can use below function, to meet your requirement

@(Pega-RULES:BusinessCalendar).addDays(<PROPERTY>, <Integer of days to add>, true (consider business calendar), OperatorID.pyCalendar (get operator calendar))

if passed from agent -- please pass OperatorID.pyCalendar as parameter.

Refer this :-

-   https://docs.pega.com/pega-services-troubleshooting/how-businesscalendar-functions-adddays-and-addtime-treat-holidays-weekends-and-time-zones

Alternatively, I have found an article containing implementation details about addToDate function:-https://collaborate.pega.com/discussion/addtodate-param-info

Function Def :-   addToDate(theDate, days, hours, minutes, seconds)

Kindly notify, if it helps.