Formula to count unique strings broken down by month

Hi AlKey

Thank you for the prompt reply.

It appeared to work perfectly on the example, however, when I tried it in the actual spreadsheet (with the ranges / references amended) I got some unexpected results.

I have attached another example (v2) which mirrors my data and has your formula in place.

As you can see the last three entries (Oct to Dec) are coming up with a fractional result. They are very close to the expected result and I have tried to trouble shoot it myself but it is approaching the end of my day and I don't think my brain can cope any more

Thanks in advance.