jqGrid not sorting dates correctly

I have an issue when sorting dates using jqGrid. Given the following dates

jqGrid is sorting my dates as follows:

01/01/2010
01/01/2011
01/02/2010
01/02/2011

I would expect to see

01/01/2010
01/02/2010
01/01/2011
01/02/2011

My date format is dd/mm/yyyy and I have the follow format options -

{
    name: 'myDate',
    index: 'myDate',
    sortable: true,
    sorttype: 'date'
}

Have I missed something here or is this a limitation of the jqGrid?