Like Excel, dates in FileMaker are stored as a number representing the number of days since an early arbitrary date and adding a number of days to a given date computes a new date that many days in the future. Thus your syntax for NextC and LeftD look correct.
I'd check to make sure that your date fields are really of type date (and that your calculation fields specify date as the result type)--not text.
Thank you, got that problem sorted out.
One thing that bugs me is with NextC calculation (modified it a bit when comparing to the first post):
If ( Recurrent = "Y" ; If ( PerformedC ≠ "" ; PerformedC + IntervalY * 365 ; Date + IntervalY * 365) ; If ( PerformedC ≠ "" ; "N/A" ; Date + IntervalY * 365 ))
Problem is with the bold part of the calculation. Basically when the bulletin is not recurrent (Recurrent ≠ "Y") and is performed (PerformedC ≠ "") it should say N/A (not applicable). BUT... since the NextC is defined as a Date field and FileMaker doesn't recognise it as a date, it only shows a question mark instead of "N/A".
Is there a way to get the field to display "N/A"?
Not and have the value returned as a date. Either convert the computed date into text: GetAsText ( Yourdate calculation here ) or define a separate field that returns the N/A. (The N/A field can be transparent and placed on top of the other field so it looks like one field to the user.) You can also place the layout text "N/A" on the layout and use conditional formatting to render it invisible when the date calculation is not empty.