How do I get the moon dial to keep the correct time?

Although I have set the moondial on my grandfather clock as instructed, it does not keep time correctly. The date of the month does not keep time either, but the clock itself if OK. I assurme that the moon dial is wrong becuase the wrong date of the month is displayed. I have set it correctly more than once but it seems to gain a couple of days each week

Tinno
January 2009