DataContractJsonSerializer parsing iso 8601 date

Use one string property for serialisation/deserialisation, and a separate, non-serialised property that converts it to a DateTime. Easier to see some sample code:

[DataContract]
public class LibraryBook
{
    [DataMember(Name = "ReturnDate")]
    // This can be private because it's only ever accessed by the serialiser.
    private string FormattedReturnDate { get; set; }

    // This attribute prevents the ReturnDate property from being serialised.
    [IgnoreDataMember]
    // This property is used by your code.
    public DateTime ReturnDate
    {
        // Replace "o" with whichever DateTime format specifier you need.
        // "o" gives you a round-trippable format which is ISO-8601-compatible.
        get { return DateTime.ParseExact(FormattedReturnDate, "o", CultureInfo.InvariantCulture); }
        set { FormattedReturnDate = value.ToString("o"); }
    }
}

You could do the parsing in the setter of FormattedReturnDate instead, to allow it to fail earlier if a bad date is received.

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Edited to include GôTô's suggestion to give the serialised DataMember the right name.