How to validate fields so that a Due date is later than a Start date?

one possible approach would be attribute customization using JavaScript. See https://jazz.net/wiki/bin/view/Main/AttributeCustomization and https://jazz.net/library/article/1093 Lab 5 for how to use attribute customization in general and using Java Script.

I have done that kind of validation already and it works.

Here example script code that does some of the computation needed:

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dojo.provide("org.example.DateValidator_30_days_later");

dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes"); // To access the work item attributes
dojo.require("com.ibm.team.workitem.api.common.Status"); // To create a status object to return
dojo.require("com.ibm.team.workitem.api.common.Severity"); // To create a severity for the status
dojo.require("dojo.date"); // We need the date class from Dojo to compare two dates
dojo.require("dojo.date.stamp"); // We need the stamp class to work with ISO date strings

(function() {
var Severity = com.ibm.team.workitem.api.common.Severity;
var Status = com.ibm.team.workitem.api.common.Status;

var DateValidator = dojo.declare("org.example.DateValidator_30_days_later", null, {

    validate: function(attributeId, workItem, configuration) {
    
        var severity= "ERROR"; 
        var message= "Date must be at least 30 days from now"; 
       
       // Work Item new: not initialized - provides todays date. Once created provides the creation date 
        var beginDate = dojo.date.stamp.fromISOString(workItem.getValue("com.ibm.team.workitem.attribute.creationdate")); 
        // Get the current attribute's value and make a Date object from it
        console.log("I got: ["+workItem.getValue(attributeId)+"]");
        var endDate= dojo.date.stamp.fromISOString(workItem.getValue(attributeId));        
        // Compare the two dates returns negative value due to order
        var compare = -(dojo.date.difference(endDate, beginDate, "day"));
        if (compare>= 30) { // make sure endDate is not earlier than beginDate + 30
            return Status.OK_STATUS;
        } else {
            return new Status(Severity[severity], message + " current value: [" +endDate + "] and difference is " + compare);
        }
    }
});
})();

Please note, you need to activate the operational behavior to prevent from saving. See the workshop and the other link.