jsonDateFormat.dwl

%dw 2.0
input records application/java
output application/json
--- 
{
  users: records map (record) -> 
    record 
      filterObject (not $$ ~= "Id")
      update {
        case createdDate at .CreatedDate -> ((createdDate >> "UTC") as String { format: "hh:mm:ss a, MMMM dd, uuuu" })
      }
}

JsonDateFormatTest.cls

List<Contact> data = [SELECT FirstName, LastName, CreatedDate FROM Contact ORDER BY LastName ASC LIMIT 5];
Assert.isFalse(data.isEmpty(), 'Contacts required for input data');

DataWeave.Script script = new DataWeaveScriptResource.jsonDateFormat();
DataWeave.Result result = script.execute(new Map<String, Object>{ 'records' => data });

// Note that DW uses Java 8 java.time.format, so uuuu is the year. vs. yyyy for Apex. 
String expected = '{\n'+
'  "users": [\n'+                                                                                                                      
'    {\n'+
'      "FirstName": "John",\n'+
'      "LastName": "Doe",\n'+
'      "CreatedDate": "' + data[0].CreatedDate.formatGMT('hh:mm:ss a, MMMM dd, yyyy') + '"\n'+
'    }\n'+
'  ]\n'+
'}';

Assert.areEqual(expected, result.getValueAsString(), 'Expected custom DateTime format in UTC');