x = DateDiff("d", DateValue("25.12.09"), Now)

Function DD(d1 As Date, d2 As Date) As String
Dim Days As Integer
Application.Volatile True
Days = DateDiff("d", d1, d2)
DD = Days & " " & Format(d2 - d1 - Days, "hh:nn:ss")
End Function

Function SumDDFromString(rng As Range) As String
Dim d As Double, dt As Long
Dim h As Long, m As Long, s As Long
Dim aTMP, aT
Dim oCell As Range
Application.Volatile

For Each oCell In rng
    If oCell Like "*# ##:##:##" Then
        aTMP = Split(oCell)
        d = d + CInt(aTMP( 0 ))
        aT = Split(aTMP( 1 ), ":")
        h = h + CInt(aT( 0 )): n = n + CInt(aT( 1 )): s = s + CInt(aT( 2 ))
    End If
Next
d = (d *  24  *  3600  + h *  3600  + n *  60  + s) /  24  /  3600 
dt = DateDiff("s",  0 , d - Fix(d))
h = dt \  3600 : m = (dt - h *  3600 ) \  60 : s = dt - h *  3600  - m *  60 
SumDDFromString = Fix(d) & " " & Format(h, "00") & ":" & Format(m, "00") & ":" & Format(s, "00")
End Function