renderer: function(date){
    // process you date to your required format and return it
}

Ext.define('App.model.DataWithDate', {
    extend: 'Ext.data.Model',

    fields: [
//...
        {
            name: 'yourDate',
            type: 'date', //or Ext.data.Types.DATE
            dateFormat: 'time' //if you have standart json object with date value like timestamp 1387481693994
        }
//...
    ]
});

{
    columns: [
        {
            header: 'Date',
            dataIndex: 'yourDate',
            xtype:'datecolumn',
            renderer: Ext.util.Format.dateRenderer('d/m/Y H:i')
        }
    ]
}