Please answer a question about the following article about Szlachta:

Around the 14th century, there was little difference between knights and the szlachta in Poland. Members of the szlachta had the personal obligation to defend the country (pospolite ruszenie), thereby becoming the kingdom's most privileged social class. Inclusion in the class was almost exclusively based on inheritance.

Around what time were the knights and szlachta very similiar?
Around the 14th century