When calculating months and hours of experience, a unique (non-overlapping) month is a month where only one project is counted. For example:
Project Dates
Project Length
Project Hours
Project 1:  01/03 to 06/03
6
1000
Project 2:  03/03 to 07/03
5
500
Subtotal
11
1500
Less Overlap (03/03 to 06/03)
4

Total Months/Hrs Counted
7
1500
As you see from this example, the months where the projects overlap are counted once. Additionally, all of the hours worked on both projects are counted. The months of experience do not have to be consecutive.