sorting not working on date column in grid

Sorting not working properly for date column in a grid

{

xtype: 'panel',

frame: false,

border: true,

widthRatio: 0.70,

flex: 1,

layout:'fit',

autoScroll: true,

items :[{

xtype: 'gridpanel',

store: 'RFQInfoGridStore',

width: '100%',

itemId: 'RFQInfoGridId',

flex: 1,

columns:[

{

text: 'Respond By Date',

flex: 1,

dataIndex: 'rfqResponseDueDate',

xtype: 'datecolumn',

dateFormat: 'Y-m-d',

sortable: true

}],

}

---- Following is the data model----

Ext.define('GPS.model.RFQInfoGridModel', {

extend: 'Ext.data.Model',

fields: [

{name:'rfqResponseDueDate', type:'date', dateFormat:'Y-m-d'}

]

});

Value that is returned from server in JSON response is 2014-10-01

sorting not working on date column in grid