Overriding datetime parsing in XmlSerializer without changing generated code

I am accessing a third party API and retrieving some XML. I was also able to access the xsd for it and I use the xsd to generate objects for deserialization using XmlSerializer.

The XML contains dates like this:

<modified>2014-08-19T06:39:13.269-0400</modified>

The generated code looks like this:

    [System.Xml.Serialization.XmlElementAttribute(DataType="time")]
    public System.DateTime modified {
        get {
            return this.modifiedField;
        }
        set {
            this.modifiedField = value;
        }
    }

Unfortunately, this results in errors when attempting to deserialize.

I can work around this by marking it with a XmlIgnore attribute and adding code like this to a partial class:

    [XmlElement("modified")]
    public string modifiedAsString
    {
        get { return this.modified.ToString(); }
        set { this.modified = System.DateTime.Parse(value); }
    }

However, the ignore part is a change to the generated classes and I'd have to make hundreds of these changes to account for all of the datetimes.

I tried the MetadataType thing to apply the attribute to it from the outside but unfortunately XmlSerializer doesn't pick that up.

Does anyone have a suggestion on how I can parse this without changing the generated code or the input XML (which is out of my control)?