Hi Nicola,
Unfortunately, the "date" command only shows the current data. More specifically, it displays your system's date, and the date format is determined by your Windows date/region settings.
You will need to do some PressTalk in order to take the format of "date", and add days to it. Fortunately, we have plenty of premade PressTalk functions that you can use to play with date formatting, available on this page:
http://planetpress.objectiflune.com/en/suite/resources/supportScroll down to the bottom of the page, and find the Libraries tab in the "Downloadable Resources" section. The "date functions" package is the one you want. Unzip it in a folder, then in PlanetPress, go to Tools->Global Function Library Manager. Go to File->Manage Folders, and select the folder that contains the library. You will now be able to import all the functions in your document by dragging them from "Libraries" to "Global functions in document". It is best to import all functions, even if you don't need all of them, because there can be dependencies between the functions. Once they are imported, you will see them in the document's global functions. You can double-click on them to open them, and check their code, along with some information on how to use them.
Finally, here's an example of a custom data selection that will display today's date + 7, using "AddDaysToDate":
=@AddDaysToDate(date(false), 'YYYY-MM-DD', 7)
This will turn "2017-07-17" into "2017-07-24". As I was saying, the format returned by "date" is dependent on your system's date settings, so your date format might be different. You may have to adjust the "YYYY-MM-DD' part of the code to match your system's date format.
Hope that helps!
Regards,
Raphaël Lalonde-Lefebvre