defining date in json file

I suggest that you consider using the JSON-js (json2.js) parser, because it parses all standard JSON, but also allows you to add custom parse handling logic, called a reviver function, which fits your scenario very well. The basic syntax to invoke the JSON parser with a custom handler looks like this:

var myObject = JSON.parse(myJSONtext, reviverFunction);

Using your example input as a guide, it could be set up to work like this:

var jsonTxt = '[{' +
        '"data1": "foo",' +
        '"Date":  "",' +
        '"childs": [{' +
            '"data2": "stack",' +
            '"Date": ""' +
        '}{}{}...]}]';    //and-on-and-on as in your comment

myData = JSON.parse(jsonTxt, function ( key, value ) {
    if ( key === 'Date') { return new Date(); }
    //any additonal custom logic you may need later...
});

A general introduction to JSON-js is provided at the JSON in JavaScript page, along with some brief intro info about JSON, and the page also includes some usage scenarios.