Identifying dates that are not included

I am trying to identify gaps in coverage for insurance. Even if there is a 1 day gap, I would like to identify that person in Column D by indicating "Gap in coverage".

For example, Joseph's coverage years are 7/7/11 to 7/6/12 (Row 2), 5/12/10 to 5/11/11 (Row 4), 7/7/12 to 7/6/13 (Row 5) and 5/12/09 to 5/11/10 (Row 7). Joseph has had continuous coverage except between 5/12/11 to 7/6/11. I would like Column D to indicate when the person had a lapse in coverage. Please assist with a simple formula - thanks