// this should the last messages that you have read. 
// if it does not exist, use DateTimeOffset.Now.ToUnixTimeSeconds(), i.e. current unix time stamp.
// var lastSeenTime = result.Messages[0].Created
var result = Message.GetMessages("token with GetMessages permission", new GetMessagesModel { Channel = "news", Time = lastSeenTime } );
if (!Helpers.IsSuccessful(messages))
{
    // we could not check for new message
    Console.WriteLine("Sorry, we could not access new messages");
}
else if (result.Messages.Count > 0)
{
    // return all new unseen messages
    for(int i = 0; i < result.Messages.Count; i++)
    {
        Console.WriteLine(result.Messages[i].Content);
    }
    // save this date and use it when calling 
    //result.Messages[0].Created
    
}
else
{
    // No new messages
    Console.WriteLine("No new messages available");
}
Console.Read();