Dates and the frequency in a rolling year

I am applying the provision of a sickness policy into a worksheet where every episode of sickness is recorded and that the first date of each period sickness is entered. As I am also applying historical data not all of the dates are in chronological order.
I can list all of a staff members dates by way of a dynamic list, that list will be in cells a5:a15
I need the spreadsheet to look for the most recent episode and then check the historical data to tell me in one cell if this most recent episode is:
The 1st episode in 6 months – “1st in 6”
The 2nd episode in 6 months – “2nd in 6”
The 3rd episode in 6 months – “3rd in 6”
I also need it to be able to tell me if there have been 4 episodes in a year (with 2 episodes falling within two separate 6 month periods) – “4th in 12”
Any ideas