Excel validation rule help with date ranges

Thanks for the quick response Dilip!
I noticed that when I added 2 more conditions (6 & 7 - see attached xlsx), the formula failed:
Any idea what I'm doing wrong?


NO. Account Sub-account Start Date End Date Assignment CONFLICT?
1 PET DOG 1/1/2012 1/31/2012 HOUSE1
2 PET DOG 1/15/2012 1/30/2012 HOUSE1 No
3 PET DOG 2/1/2012 2/29/2012 HOUSE3 No
4 PET CAT 2/1/2012 2/29/2012 HOUSE3 No
5 PET DOG 1/1/2012 1/31/2012 HOUSE2 Yes
6 PET CAT 2/15/2012 2/29/2012 HOUSE1 No <--Should fail, since conflicts with No. 4.
7 PET CAT 2/15/2012 3/15/2012 HOUSE2 No <--Should fail, since conflicts with No. 4.