JSFiddle - React, Tailwind, and code Playground

JavaScript

var new_object = {
    '0300' : ["295", "293", "298"],
	'0800' : ["293", "298"],
	'0930' : ["295"],
	'1130' : ["297", "293", "298"],
	'1230' : ["295"]
};

var old_object = {
	'0300' : ["297"],
	'0800' : ["297"],
	'0930' : ["293"]
};

function merge_keys(o1, o2, merge) {
  var keys1 = Object.keys(o1);
  var keys2 = Object.keys(o2);
  var key;

  for (var i = 0; i < keys2.length; i++) {
    key = keys2[i];
    if (keys1.indexOf(key)) o1[key] = merge(o1[key], o2[key])
    else o1[key] = o2[key]; 
  }
}

function concat(a, b) { return a.concat(b); }

merge_keys(new_object, old_object, concat);

console.log(new_object);