My GTD "Update" script calculates next review dates from a column "Recur" in the notebook.
However, sometimes I want to set that date myself. AppleScript provides the "days" and "weeks" constants, which you can use to calcululate future dates, but it does not have "months" and "years", so a script must calculate those dates itself.
Here's a short subroutine that will do this. It is derived from a script in this thread at MacScripter.