You are sorting Review objects, not datetime objects. Use a review comparer like this.
Public Class ReviewComparer
Implements IComparer(Of Review)
Public Function Compare(xr As Review, yr As Review) As Integer Implements System.Collections.Generic.IComparer(Of Review).Compare
Dim x As DateTime = xr.reviewdatetime
Dim y As DateTime = yr.reviewdatetime
If x = Nothing Then
If y = Nothing Then
Return 0
Else
Return -1
End If
Else
If y = Nothing Then
Return 1
Else
Dim retval As Integer = x.CompareTo(y)
If retval <> 0 Then
Return retval
Else
Return x.CompareTo(y)
End If
End If
End If
End Function
End Class