Hi,
here the code:
var test = test || {};
test.Customizing = {
Load: function () {
Xrm.Page.getAttribute("your-field-with-value").addOnChange(this.AddYear);
},
AddYear: function () {
var fieldBValue = Xrm.Page.getAttribute("your-field-with-value").getValue();
var fieldCValue = new Date(fieldBValue.getFullYear() + 5, fieldBValue.getMonth(), fieldBValue.getDate());
Xrm.Page.getAttribute("your-field-to-update").setValue(fieldCValue);
}
};
add it to js file. Register it on Form to OnLoad Event.
Greetz