[Koha] Serials: two per month?

I'm setting up serials this week, and I don't see a frequency of two issues per month. I see one issue every two weeks, but that leaves me with two extra each year. Am I missing something right in front of me, or should I submit an enhancement request? And if it's not there, can anyone recommend a workaround? Thanks, Fred King kohauser@phred.us