Imports System.Net
Imports System.Xml
Imports System.String

Class Rates
    Public DayDate As String
    Public ExchRate As Double

    Public Sub New(ByVal _DayDate As String, ByVal _ExchRate As String)
        DayDate = _DayDate
        ExchRate = _ExchRate
    End Sub

End Class

Class RatesComparer
    Implements IEqualityComparer(Of Rates)

    Public Function Equals1(Rate1 As Rates, Rate2 As Rates) As Boolean Implements System.Collections.Generic.IEqualityComparer(Of Rates).Equals
        Return (CDate(Rate1.DayDate) = CDate(Rate2.DayDate))
    End Function

    Public Function GetHashCode1(Rate1 As Rates) As Integer Implements System.Collections.Generic.IEqualityComparer(Of Rates).GetHashCode
        Return Rate1.DayDate.GetHashCode()
    End Function

End Class

Public Class Form1
    Private Const ECB_URL As String = "http://www.ecb.europa.eu/stats/eurofxref/eurofxref-hist-90d.xml"
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Dim doc = XDocument.Load(ECB_URL)
        Dim gesmes As XNamespace = "http://www.gesmes.org/xml/2002-08-01"
        Dim ns As XNamespace = "http://www.ecb.int/vocabulary/2002-08-01/eurofxref"
        Dim Root = doc.Element(gesmes + "Envelope")
        Dim query = (From nodelist In Root.Elements(ns + "Cube").Elements(ns + "Cube").Descendants _
                    Where CDate(nodelist.Ancestors.Attributes("time").Last.Value) > CDate("2012-06-01") _
                    Order By nodelist.Ancestors.Attributes("time").Last.Value Ascending _
                    Select New With {.t = nodelist.Ancestors.Attributes("time").Last.Value, .r = CDbl(1)}).Distinct(New RatesComparer())
        Debug.Print(query.Count())
    End Sub
End Class

Dim query as ...
query = ...

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Dim doc = XDocument.Load(ECB_URL)
        Dim gesmes As XNamespace = "http://www.gesmes.org/xml/2002-08-01"
        Dim ns As XNamespace = "http://www.ecb.int/vocabulary/2002-08-01/eurofxref"
        Dim Root = doc.Element(gesmes + "Envelope")
        Dim query As List(Of Rates) = (From nodelist In Root.Elements(ns + "Cube").Elements(ns + "Cube").Descendants _
                    Where CDate(nodelist.Ancestors.Attributes("time").Last.Value) > CDate("2012-06-01") _
                    Order By nodelist.Ancestors.Attributes("time").Last.Value Ascending _
                    Select New Rates(nodelist.Ancestors.Attributes("time").Last.Value.ToString, CDbl(1))).ToList()

        Debug.Print(query.Distinct(New RatesComparer()).Count())
    End Sub