To Do task that recurs every x interval

I'm currently building a to-do list (I know there are existing gems but I'm doing a tailored fit feature for a specific project).

One of the biggest issue I'm having is creating the feature like in Google calendar where you can create a to-do task that will show up every x days. I can easily calculate this on the current month since there's start date but the problem is when I proceed to the succeeding months (the user can move forward ahead so I can't save the last day from previous month) I lose track of when the dates should properly appear.

If anyone can point me to the right direction or give a basic formula for this I would gladly appreciate it.