Are you working with the source code for ViPR Data Service SDK?  There is an issue in the S3Object class with hose the lastAccessed property is getting formatted.  I was able to get it to work with the following tweak:

this.lastModified = DateTime.ParseExact(

                    value,

                    "yyyy-MM-ddTHH:mm:ssZ",

                    //new string[] { AWSSDKUtils.ISO8601DateFormat, AWSSDKUtils.GMTDateFormat },

                    CultureInfo.InvariantCulture,

                    DateTimeStyles.None

                    );

However, is there any specific reason you're using this specific SDK as it's quite outdated.  Can you instead reference the Amazon.S3 nuget package directly in your solution and work with the object store through that SDK instead?  Please use the link below to locate the AWS SDK using .NET.

Getting Started with ECS SDKs

Thanks,

Ben