Excel date format inconsistencies

I frequently need to convert columns of dates in Excel formatted as dd.mm.yyyy to dd/mm/yy. If I perform a simple replace '.'with '/' all is well and the date formats are correct, however if I record a macro to do this the resulting formats are mixed (i.e. some are dd/mm/yy and some are mm/dd/yy). My question is twofold;

1) Why does this happen?
2) How can I stop this happening?