Calculate end date from start date and No. of month in nav

Hi team,

I am working on page which contains Start Date, End Date and No. of Month. Am trying to write a code that when I inputted the Start Date (Date), No. of Month (Integer) , it should give End Date.

Start Date - OnValidate()
IF End Date" <> 0D THEN
  VALIDATE( End Date");

End - OnValidate()
IF "End Date" > "Start Date" THEN
"No. of Months" := ROUND(("End Date"-"Start Date")/30,1)
"No. of Months" := 1;

No. of Month - OnValidate()

The above code worked for the calculate of No. of Month but I need for the End Date.