Adding/subtracting days from date?

I have a FormAssembly form where a user is inputting in a date. I'd like to be able to manipulate that date in a couple of ways, but I'm unsure if I'm able to do so in FormAssembly. I'd like to be able to:

• Construct a new date out of the entered date (based on the entered date, set it to the first of the next month for example) 
• Add/subtract days from that date

I'm not seeing a clear way to do this with the formulas available. This date is also in a repeatable section so I don't think that I can use form calculations either. Has anyone had luck with this? @Nonprofit Success Pack  @Nonprofit Hub