time1 As String
time2 As String
nMin As Long
 
time1 = "02:00"
time2 = "04:00"
' time2 in Minuten darstellen
nMin = Val(Left$(time2, 2)) * 60 + Val(Mid$(time2, 4))
 
' jetzt die Minuten von time1 abziehen
Debug.Print Format$(DateAdd("n", -nMin, time1), "hh:nn")

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