I have now released version 1.12 of the ordernumber plugin, which allows the admin to configure in more detail when the counter is "reset" (internally, it is not really reset, but a new counter with a new name is started). In your case, the proper number format to have the counter reset yearly would be:
[day]-[month]-[year] #|[year] #
Of course, you'll have to enable "Separate counter per format value". (Notice, however, that if you already have orders, the counter would start from 1 again).
So, you probably have your shop set up now and don't want to change this. However, I'm posting this example in case someone else with a similar issue reads through our support forum and reads this thread.
Best regards,
Reinhold