I ended up using this JavaScript code to add a user-defined number of years to a user-defined date:
var execDate = myDateField.rawValue;
//tell JS that years is a number
yearsToAdd = yearsToAdd * 1;
//convert to JS format
execDate = execDate.replace(/-/g,"/");
var termDate = new Date(execDate);
//add years and subtract one day
termDate.setFullYear(termDate.getFullYear() + yearsToAdd);
termDate.setDate(termDate.getDate() - 1);
//insert the date
myTargetDateField.formattedValue = util.printd("mmmm d, yyyy", termDate);
also see this thread
http://www.adobeforums.com/cgi-bin/webx/.3bc20e91/0