How to convert an ISO string to the date object?

https://jazz.net/wiki/bin/view/Main/AttributeCustomization

The api explains how to use the conversion to dojo.date/dojo.date/stamp

That is all you really need. For how to work with that I would suggest to search the web.

Conversion to other date see https://www.w3schools.com/jsref/jsref_obj_date.asp

Here an example how I use the dojo objects

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

dojo.require("com.ibm.team.workitem.api.common.Severity");
dojo.require("com.ibm.team.workitem.api.common.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

/
 
  You need to configure this script in the Process Configuration Source:
      <validator id="dateval" name="Date Validator" providerId="com.ibm.team.workitem.shared.common.internal.valueProviders.ScriptAttributeValueProvider">
          <script class="org.example.DateValidator" path="/workitem/scripts/common/DateValidator.js"/>
          <parameters beginDateAttributeId="startdate" message="The completion date can not be earlier than the start date" severity="ERROR"/>
      </validator>
 
 */
(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", null, {

    validate: function(attributeId, workItem, configuration) {
    
        // Get the configuration parameters about the severity and error message of this validator
        var severity= configuration.getChild("parameters").getStringDefault("severity", Severity.ERROR.name);
        var message= configuration.getChild("parameters").getStringDefault("message", "");
        
        // Get the begin date attribute from the configuration and make a Date object from it
        var beginDateId = configuration.getChild("parameters").getStringDefault("beginDateAttributeId", "");
        var beginDate = dojo.date.stamp.fromISOString(workItem.getValue(beginDateId));
        
        // Get the current attribute's value and make a Date object from it
        var endDate= dojo.date.stamp.fromISOString(workItem.getValue(attributeId));
        
        // Compare the two dates and make sure endDate is not earlier than beginDate
        if (dojo.date.compare(endDate, beginDate) >= 0) {
            return Status.OK_STATUS;
        } else {
            return new Status(Severity[severity], message);
        }
    }
});
})();