Maintenance Plans can be created for the maintenance activities which are planned well in advance (atleast annually) with predefined frequencies.
In your case, its 6 months frequency. You have the option of creating on specific date by selecting the option as Time Key date in maintenance plan. So as per your scenario (for 6 month), if you want to create Maintenance Order on 10-Dec-2013, then mention the start of cycle as 10-Jun-2013. So that, orders will be generated on 10th of 6 month of all cycle.
5 month unplanned maintenance can be done by doing Manual Call in IP10 which will be happened on 6th month. By this, you can advance or postpone the creation of order by manual intervention.