Dates are actually special purpose number fields that store an integer counting the number of days from 12/31/0000 to the date recorded in that date field. You can thus treat them like number fields in calculations, but if the result is a date, you need to specify "date" as the return type.
If MaintDate is a field of type Date and MaintInterval is a field of type number, then you can define cNextMaintenance as:
MaintDate + MainInterval
just be sure to select Date in the return type drop down or you'll see a large integer value (That number of days from 12/31/0000), instead of a date.