Option Explicit
 
Private Sub Form_Load()
MsgBox SumarTiempo("00:02;20.254", "00:02;59.800")
End Sub
 
Private Function SumarTiempo(Tiempo1 As String, Tiempo2 As String) As String
Dim H As Integer, H2 As Integer
Dim M As Integer, M2 As Integer
Dim S As Integer, S2 As Integer
Dim Ml As Integer, Ml2 As Integer
Dim Horas As Integer, Minutos As Integer, Segundos As Integer, Milesimas As Integer
 
H = Mid(Tiempo1, 1, 2)
M = Mid(Tiempo1, 4, 2)
S = Mid(Tiempo1, 7, 2)
Ml = Mid(Tiempo1, 10, 3)
 
H2 = Mid(Tiempo2, 1, 2)
M2 = Mid(Tiempo2, 4, 2)
S2 = Mid(Tiempo2, 7, 2)
Ml2 = Mid(Tiempo2, 10, 3)
 
Horas = H + H2
Minutos = M + M2
Segundos = S + S2
Milesimas = Ml + Ml2
 
If Milesimas >= 1000 Then
Segundos = Segundos + 1
Milesimas = Milesimas - 1000
End If
 
If Segundos >= 60 Then
Minutos = Minutos + 1
Segundos = Segundos - 60
End If
 
If Minutos >= 60 Then
Horas = Horas + 1
Minutos = Minutos - 60
End If
 
SumarTiempo = Format(Horas, "00") & ":" & Format(Minutos, "00") & ";" & Format(Segundos, "00") & "." & Format(Milesimas, "000")
 
End Function