Imports System
Imports System.IO
Imports System.Collections
Imports System.Collections.Generic
Imports System.Runtime.Serialization
Imports ServiceStack
Imports ServiceStack.DataAnnotations
Imports Test.ServiceModel
Namespace Global
    Namespace Test.ServiceModel
        Public Partial Class HelloDateTime
            Public Overridable Property DateTime As Date
        End Class
    End Namespace
End Namespace
To override the Content-type in your clients, use the HTTP Accept Header, append the .csv suffix or ?format=csv
The following are sample HTTP requests and responses. The placeholders shown need to be replaced with actual values.
POST /csv/reply/HelloDateTime HTTP/1.1 
Host: test.servicestack.net 
Accept: text/csv
Content-Type: text/csv
Content-Length: length
{"dateTime":"\/Date(-62135596800000-0000)\/"}
HTTP/1.1 200 OK
Content-Type: text/csv
Content-Length: length
{"dateTime":"\/Date(-62135596800000-0000)\/"}