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

Dave, I think it's because the formula you are using does not take into consideration the entire date ranges as a whole. For example, if there are dates that overlaps, but are not in adjacent cells. For example, if you swap the last 2 dates at the bottom of the OPs example, the result would not be correct.

Row\Col	A	B	C	D
1	Start date	End date
2	06-Jan	10-Jan	0	=SUM(IF(FREQUENCY(ROW(INDIRECT($A2&":"&$B2)),IFERROR(ROW(INDIRECT($A1&":"&$B1)),0))=1,1,0))+$C1
3	13-Jan	20-Jan	0
4	18-Jan	25-Jan	3
5	12-Jan	22-Jan	7