No Access o seu campo Data/Hora,deixa o formato da Hora como Abreviada.

Dim totalhours As Long, totalminutes As Long
Dim days As Long, hours As Long, minutes As Long
Dim interval As Variant, j As Integer

RS1.Open "SELECT Clientes.Hora, Clientes.CLIData From Clientes WHERE Clientes.CLIData Between
#" & Format(Text1.text, "DD/MM/YYYY") & "# AND #" & Format(Text2.text, "DD/MM/YYYY") & "# And
Clientes.Codigo=" & txtCodigo.Text & ";"

interval = #12:00:00 AM#
While Not RS1.EOF
interval = interval + RS1![HORA]
RS1.MoveNext
Wend
totalhours = Int(CSng(interval * 24))
totalminutes = Int(CSng(interval * 1440))
minutes = totalminutes Mod 60
hours = totalhours Mod 24

Label1.Caption = totalhours & ":" & minutes

Acredito que seja isto.
Achei estes codigo no site do Bill.