Thank you for your answer.
I figured that "our" months are equivalent to the DnD months, so Hammer/Deepwinter (January) = 1, Alturiak/Claw of Winter (February) = 2, and so on. I still don't know how the Calender of Harptos changes the numbers, for me it only changes the names of the months but not their order.
Midsummer is between 7 and 8 (July/August - Flamerule/Eleasias), Midwinter between 1 and 2 and Highharvestide between 9 and 10. Either way i don't get a 5-number Code (and i cant enter 10 because i dont have a 0). So I either don't get the numbers of the month or i have the wrong months. However, since there are only 12 Months in the Harptos calender as well, i figured that I need 11 and 12 just because these are the only two-number Months which could get me a 5 number code...
I especially want to know what my mistake is - if that would spoil the solution i would appreciate a PM. Thank you