monthly counter and 2-digit year for invoice numbering

we need to have our invoice numbers like this:

{$monthlyCounter}{$date:m}{$date:y}{$counter}
how can i achieve that?

{$monthlyCounter} should be a counter per month ("07" for the 7th invoice in march)
{$date:m} should be a 2-digit month (seems to work already)
{$date:y} should be a 2-digit year
{$counter} should be a 2-digit counter (seems to work already)

To Summarize my Question:

1. Why is the 2-digit Year variable $date:y not working?
2. How do i integrate an additional independent monthly counter?