I am using Ext-js 3.2, in that i face a problem in datefield with a particular format 'M/Y'

Here my code,
{xtype:'datefield',fieldLabel: 'Month/Year ', name: 'monyear', allowBlank: false, format:'M/Y',width:100}

when i choose NOV month it automatically goes to DEC month on (system current date is OCT 31).

if i change the datefield format to some thing else, the thing is correct.

Please help me to solve this problem. thanks in advence...