Utiliza la siguiente funcion:
Public Function HoraMilitar(ByVal xHora As String) As String
xHora = UCase(xHora)
largo = Len(xHora)
pos1 = InStr(xHora, ":")
laHora = CInt(Mid(xHora, 1, pos1 - 1))
pos2 = InStr(xHora, " ")
LosMinutos = CInt(Trim(Mid(xHora, pos1 + 1, pos2 - pos1)))
pos3 = InStr(xHora, "P")
If pos3 > 0 Then
If laHora <> 12 Then
laHora = laHora + 12
End If
End If
HoraMilitar = Format(laHora, "00") & ":" & Format(LosMinutos, "00")
End Function