Handler for when the DatePicker for a field fires the 'select' event
@param {Ext.picker.Date} picker
@param {Object} date
function (picker, date) {
        var fields = this.fields,
            field = this.fields[picker.itemId];

        field.setChecked(true);

        if (field == fields.on) {
            fields.before.setChecked(false, true);
            fields.after.setChecked(false, true);
        } else {
            fields.on.setChecked(false, true);
            if (field == fields.after && this.getFieldValue('before') < date) {
                fields.before.setChecked(false, true);
            } else if (field == fields.before && this.getFieldValue('after') > date) {
                fields.after.setChecked(false, true);
            }
        }
        this.fireEvent('update', this);

        picker.up('menu').hide();
    }