Fix an online calendar built on Zend Framework

I have an online events calendar which displays month-by-month upcoming events at a school. Unfortunately every alternate month the calendar has a fault which means the user can only view every 2nd month - for example, clicking on October would display November, clicking on December would display January etc.
The calendar works as it should just now (September) but in October the issue will re-occur - I'd like to have the problem fixed before then if possible.