:Suppose Ivan Ivanovitch was born 6 January 1813 NS 25 December 1812 OS. 

 :As I see it there are four issues here: 
 :# Will the "born in 1812" category include Ivan wrongly? 
 :# Will the 1812 article include Ivan Iavanovitch on the OS birthday? 
 :# Will the 25 December page include Ivan? 
 :# Will it look OK? 

 :# I don't think so, the process is smarter than that, and wiifying the date certainly won't affect it. 
 :# No, that's a manual process (or the page wuld be overloaded). 
 :# Ditto. 
 :# Yes, I think because otherwise the date will look something like 6 January 1813 NS December 25 1812 OS to some users. 

 :   Rgds, 
 :        ''''