Calculate date with different intervals

Hi,

I hope im posting this in the correct board

first of all I'm using Sharepoint 2013 and Nintex Workflow / Forms 2013.

I have following requirement:

I'm currently working on a SharePoint List + Nintex Form + Nintex Workflow combination.

The user should be able to choose a "start date" and an interval for the "next date" in the form.

Intervals should be something like:

- daily

- "Weekday" every "X" Weeks (like "Wednesday" every "3" Weeks)

- The first "Weekday" of the month (like The first "Monday" of the month)

Do you guys know any easy way to calculate all of these intervals in the workflow?

The outcome should be a date variable.

BR