Sencha Touch 2: convert function in model modifies record in store

Question

I've created a store with a restproxy and a list which is using the PullRefreshPlugin. In my model I have a convert function for a datetimefield. When I refresh my list (via plugin) all my records have modified fields.

I think this is caused by the convert function in my model. Is there any way to avoid it?

Thanks in advance

Edit: Codesnippet

Model:

Ext.define('MyApp.model.MyModel', {
    extend: 'Ext.data.Model',

    config: {
        fields: [
            {
                name: 'dateTimeField',
                type: 'string',
                convert: function (value) {
                    if (value) {
                        if (Ext.isDate(value)) {
                            return Ext.util.Format.date(value, 'c');
                        } else if (Ext.isString(value)) {
                            var match = value.match(/(.+)\[.+\]/);
                            if (match && match[1]) {
                                return Ext.Date.parse(match[1], 'c');
                            }
                        }
                    }
                }
            }
        ]
    }
});

Store:

Ext.define('MyApp.store.MyStore', {
    extend: 'Ext.data.Store',
    requires: ['Ext.data.proxy.Rest'],

    config: {
        model: 'MyApp.model.MyModel',
        proxy: {
            type: 'rest',
            url: '/path/to/items',
            reader: {
                rootProperty: 'root',
                totalProperty: 'totalElements'
            }
        }
    }
});

List:

Ext.define('MyApp.view.MyList', {
    extend: 'Ext.dataview.List',
    requires: ['Ext.plugin.ListPaging', 'Ext.plugin.PullRefresh'],

    config: {
// config stuff..
        plugins: [{
            xclass: 'Ext.plugin.ListPaging',
            autoPaging: true
        }, {
            xclass: 'Ext.plugin.PullRefresh',
            lastUpdatedDateFormat: 'd.m.Y, H:i:s'
        }]
    }
});

In my controller I'm just using store.sync()