por cierto, para vb.net quedaría así:
Código:
Public Function SumarHoras(ByVal h1 As String, ByVal h2 As String) As String
Dim arrHour1() As String = h1.Split(":")
Dim arrHour2() As String = h2.Split(":")
Dim d1 As DateTime = New DateTime(1900, 1, 1, Int32.Parse(arrHour1(0)), Int32.Parse(arrHour1(1)), 0)
d1 = d1.AddHours(Int32.Parse(arrHour2(0)))
d1 = d1.AddMinutes(Int32.Parse(arrHour2(1)))
Return d1.ToShortDateString
End Function