Patek Philippe

Good day. Would anyone care to explain how to reset the day/night indicator on the 5960. I recently bought one from a friend. Upon wearing it for a few days, I realized that the indicator is not in sync with the main time (does not turn blue at 6:00PM and... 

from 6 a.m. to 10 a.m. it changes from blue to white. Stays white till 6 p.m. and then changes to blue until 10 p.m. Theres no way to adjust it...

Thanks p.boxster. In that case, mine is working perfectly then.