How to find Sum of # of days w/o counting overlapping days twice

Hmm, try this.

I've added an additional helper column to determine whether a row is hidden or not in column C.

Row\Column	A	B	C	D	E
1	Start date	End date	Duration	Not hidden?	Duration (no overlap)
2	6-Jan	10-Jan	5	1	19
3	13-Jan	20-Jan	8	1
4	12-Jan	22-Jan	11	1
5	18-Jan	25-Jan	8	1

Formula in D2 (subsequently copied down)

Formula:

=SUBTOTAL(103,A2)

Formula in E2 (array formula)

Formula:

=SUM(1*(MMULT(((MIN(A2:A5)-ROW(A2)+ROW(OFFSET(A2,0,0,MAX(B2:B5)-MIN(A2:A5)+1,1)))>=TRANSPOSE(A2:A5))*((MIN(A2:A5)-ROW(A2)+ROW(OFFSET(A2,0,0,MAX(B2:B5)-MIN(A2:A5)+1,1)))<=TRANSPOSE(B2:B5)),D2:D5)<>0))

See also attached.