O seguinte fará o que você quer e é totalmente configurável, além de suportar qualquer formato de entrada ou saída, contanto que o Excel ainda o entenda como uma data / hora numérica. Você pode definir qualquer início ou fim para suas horas / dias de trabalho.
Public Function EndDayTimeM(StartTime As Double, Minutes As Long)
Dim rangeH, numH, rangeD, numD, startD, durW, durD, durH, durM, startW, endW, remTime As Long
Dim startH, endDate As Double
rangeH = 8 ' Starting hour of working day
numH = 9 ' Length of working day in hours
rangeD = 2 ' Starting day of working week
numD = 5 ' Length of working week in days
' Calculates offset from 00:00 Monday in starting week
startW = Fix(StartTime) - DatePart("w", StartTime)
startD = DatePart("w", StartTime) - rangeD
startH = (StartTime - Fix(StartTime)) * 24
' Calculates end time in working weeks, hours, minutes
remTime = Minutes + (startD * numH * 60) + ((startH - rangeH) * 60)
durW = Fix(remTime / 60 / numH / numD)
remTime = remTime - (durW * numD * numH * 60)
durD = Fix(remTime / 60 / numH)
remTime = remTime - durD * 60 * numH
durH = Fix(remTime / 60)
remTime = remTime - durH * 60
durM = remTime
' Converts working weeks into calendar weeks
endDate = startW + durW * 7 + rangeD + durD + (rangeH + durH) / 24 + durM / 1440
EndDayTimeM = endDate
End Function