First install this tiny UDF:

Function duration(s As String) As Integer
t = Split(s, "-")
duration = DateValue(t(1)) - DateValue(t(0))
End Function

Then, with your data in A1 thru C1, in D1 enter:
=duration(A1)+duration(B1)+duration(C1)

to see 12



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Gary''s Student - gsnu200725