JSFiddle - React, Tailwind, and code Playground

JavaScript

var DATA_TABLE = [
    {"rownum": 0, "Surname": "Doe", "Firstname": "John", "Dob": "1 Jan 1981"}, 
    {"rownum": 1, "Surname": "Sally", "Firstname": "Bram", "Dob": "1 Jan 1981"}, 
    {"rownum": 2, "Surname": "Bob", "Firstname": "Joe", "Dob": "1 Jan 1982"},
];

DATA_TABLE.sort(function(a, b) {
  return new Date(a.Dob) - new Date(b.Dob) 
  || ((a.Surname   < b.Surname)   ? -1 : (a.Surname   > b.Surname)   ? 1 : 0)
  || ((a.Firstname < b.Firstname) ? -1 : (a.Firstname > b.Firstname) ? 1 : 0) ;
});

alert(JSON.stringify(DATA_TABLE));