Date calculation

I would like a calculation of 2 date input which only gives gives me the year difference in whole year on the 2 inputs.
The inputs have the format [dd-mmm-yyyy] and the script shown below also seems OK for that, but I have 2 more wishes that I would like the calculation to perform.
If the inputs are only entered in the format [yyyy], it must also be able to calculate the difference throughout the year, and if one or both inputs are not of the formats [dd-mmm-yyyy] or [yyyy], the output must be blank (nothing).

Please help me with what the calculation should look like to fulfill my wishes.

// Custom calculate script
(function () {

var sStart = getField("Dato5").valueAsString;
var sEnd = getField("Dato6").valueAsString;
var dStart, dEnd, diff;

if(sStart && sEnd) {
dStart = util.scand("dd-mmm-yyyy", sStart);
dEnd = util.scand("dd-mmm-yyyy", sEnd);
diff = dEnd.getTime() - dStart.getTime();
event.value = Math.floor(diff / 31536e6) + " år";
} else {
event.value = "";
}

})();