>Einar's idea is great, with one suggestion. Instead of subtracting a second from midnight, it is probably safer to subtract one from the underlying Long value.

Ok, thanks, I ended up with those two functions:

    ' Return a date starting on the day
    ' expT1 Date
    Public Shared Function GetDateDayStart(ByVal tdDate As Date) As Date
        Return DateTime.Parse(Format(tdDate, "dd/MM/yyyy 00:00:00"))
    End Function

    ' Return a date ending on the day
    ' expT1 Date
    Public Shared Function GetDateDayEnd(ByVal tdDate As Date) As Date
        Return DateTime.Parse(Format(tdDate, "dd/MM/yyyy 23:59:59"))
    End Function