You can use the LastWriteTime or the LastWriteTimeUtc property to get or set the last time the file was modified. LastWriteTimeUtc displays the time in Coordinated Universal Time (UTC).
Dim infoReader As System.IO.FileInfo
infoReader = My.Computer.FileSystem.GetFileInfo("C:\testfile.txt")
MsgBox("File was last modified on " & infoReader.LastWriteTime)