GetDateTimestamp useage

[=?Utf-8?B?SGFs?=]

Hello

When
din instring as string
Dim listRequest As FtpWebRequest = WebRequest.Create(listUrl)
listRequest.Method = WebRequestMethods.Ftp.ListDirectory
Dim listResponse As FtpWebResponse = listRequest.GetResponse()
reader = New StreamReader(listResponse.GetResponseStream())
instring =reader.ReadLine()

how do I use instring with GetDateTimestamp to set the time on the file ?