Lets say you need a custom date field 30 days from today.
The formula in the custom filed is this
- Datepart(Adddate(Now(),1,'MONTH'))
But understand that whilst Adddate() returns a Date/Time, Datepart() returns a String so when you create your custom field set "Formula Return Type" to String.
I made this post because once you've created your custom field you can't change the "Formula Return Type", you have to delete the formula from your layout and if you want to re-use the same field name, from the "Unused Fields" area.
And yes, it caught me out!