How to calculate increase a date value by 7 days

Hello,

I received a requirement to increase the due date of an invoice for collections by 7 days by using the RFT template.

So my question is;

- when I receive the date in formate through xml date in format <30-07-2017>

- how I can increase this value directly in the RTF template by adding 7 days

Is it possible to add some kind of XDO function in the help like in the orintscreen below;

and change it to <?format-date:normal_date + 7?>

Looking forward to hear your feedback.

Thanks for your help in advance.

Best regards,

Wim van der Veer