Take date form field value and add days to it. Dynamics 365 Web Resource

dynamics_addDays_field.js

function addDays(date, days) {
var result = new Date(date);
result.setDate(result.getDate() + parseInt(days));
return result;
}

function recalculateDateFieldValue() {
    var date= Xrm.Page.getAttribute("effectivefrom").getValue();
    
    if (date != null) {
    Xrm.Page.getAttribute("effectiveto").setValue(addDays(date, 30));
    }
    }